The US-led coalition fighting Islamic State has begun preparing to open a major front in northeastern Syria, aiming to put pressure on Raqqa, the terrorist group's de facto capital, according to military and administration officials.

President Barack Obama last week approved two important steps to set the offensive in motion over the coming weeks, officials said. Obama ordered the Pentagon, for the first time, to directly provide ammunition and perhaps some weapons to Syrian opposition forces on the ground. He also endorsed the idea of an increased air campaign from an air base in Turkey, although important details still need to be worked out.

Together, these measures are intended to empower 3,000 to 5,000 Arab fighters who would join more than 20,000 Kurdish combatants in an offensive backed by dozens of allied warplanes to pressure Raqqa, Islamic State’s main stronghold in Syria. Plans are also moving forward to have Syrian opposition fighters seal an important 60-mile part of the country’s border with Turkey to cut off critical supply lines of Islamic State, also known as Isis or Isil.

As recently as Friday, Obama said he would take all steps necessary to combat Islamic State in Syria and Iraq. The new approach relies on Arab fighters, whose commanders have been screened by US forces, as well as Kurdish fighters who are more battle-tested and whose loyalties Washington can count on.

"The top-line message that I want everybody to understand is, we are going to continue to go after Isil," Obama told reporters. "We are going to continue to reach out to a moderate opposition." Senior administration officials say the new offensive holds promise and may change the dynamics on the ground. But it comes a year after a US-led coalition started a campaign against Islamic State that is now "tactically stalemated", Gen Martin Dempsey, then chairman of the Joint Chiefs of Staff, said last month.

Whether the new approach can succeed remains to be seen. Islamic State has proved to be more resilient to coalition attacks and adaptive in the face of international pressure than US officials anticipated, even managing to extend its reach and control inside Syria and Iraq.

The new US-led push would be conducted far from the brunt of the Russian air campaign in western Syria. That Russian operation has been largely directed at Syrian groups that oppose President Bashar al-Assad, and is only nominally aimed at Islamic State, US officials said.

The new northern front would be the opposite: It is entirely directed at weakening Islamic State by trying to take away the group’s home-court advantage, even as the militants hold on to Mosul and Ramadi in Iraq and Palmyra in Syria. The outlines of the mission have been drawn from public statements of senior commanders briefing Congress as well as interviews with more than a half-dozen military, diplomatic and administration officials who spoke on the condition of anonymity to discuss internal planning. Even in describing the goals of the campaign, officials said they would not disclose the kinds of details that might help Islamic State anticipate exactly how the planned offensive would unfold.

Gen Lloyd J Austin III, the commander of US forces in the Middle East, hinted at the emerging strategy last month, telling the Senate Armed Services Committee that over the next six months it would put "a lot more pressure on key areas in Syria, like the city of Raqqa".

"Because of that access," Austin continued, referring to the use of the air base in Turkey, "we'll have the ability to increase the pace and focus on key places in Syria. So that will certainly shake things in Iraq." Last week, Obama held a National Security Council meeting that endorsed the main elements of the strategy. At that meeting, administration officials said, Obama backed the basic idea for the Syrian Kurdish-Arab push toward Raqqa supported by United States and other coalition airpower.

On Wednesday, secretary of state John Kerry alluded to the main elements of the northern front operation at a meeting at the UN Security Council. With the Russian foreign minister, Sergei Lavrov, presiding, Kerry said: "We are now in position with France, Australia, Canada, Turkey and other coalition partners joining the campaign, to dramatically accelerate our efforts. This is what we will do."

Kerry said, “We will also be sustaining our support to anti-Isil fighters in northeast Syria.” “Isil,” he went on, “will soon face increasing pressure from multiple directions across the battlefield in Syria and Iraq.” A spokeswoman for the National Security Council, Emily Horne, declined on Sunday to comment on the mission, citing “operational security”.

The origins of the northern front lie in the fight for Kobani, the Syrian Kurdish border city that faced an Islamic State onslaught last year. Kobani showed the potential for using a combined air and ground operation to defeat Islamic State. The United States and its allies provided the combat aircraft, and Syrian Kurdish fighters, in contact with US Special Operations Forces in northern Iraq, provided the ground force.

In just a few months, that campaign not only held on to Kobani, but also routed Islamic State fighters across a stretch of territory from the Euphrates River to the Iraqi border. The operation now being prepared would expand the Kurdish effort by adding Arab groups. In addition to increasing the number of anti-Islamic State fighters, the inclusion of Arab fighters eases Turkish concerns that the Syrian Kurds are becoming too influential in northern Syria.

The Arab wing of this ground force is called the Syrian Arab Coalition, a conglomeration of 10 to 15 groups whose total numbers range from 3,000 to 5,000 fighters, US officials said. They would fight alongside a larger Kurdish force in the northeast of as many as 25,000 fighters.

US military officials have screened the leaders of the Arab groups to ensure they meet standards required by Congress when it approved $500 million last year for the Defense Department to train and equip moderate Syrian rebels. Most of the focus of that financing has been on an ill-fated Pentagon training program at sites in Turkey and Jordan that has fielded few fighters.

The administration’s plan is to support the Kurdish and Arab fighters and have them advance toward Raqqa, but not try to seize the heavily defended city itself. Rather, the aim is to isolate Raqqa and cut if off from travel and supply lines northeast and northwest of the city.

Under planning for the northern offensive, coalition airpower at Incirlik, Turkey, would also be expanded. More nations could base aircraft there. Australia, France and Turkey have all recently starting flying strike missions against targets in Syria.

By gradually expanding the area of the coalition's air operations, the administration could protect more US-backed rebel forces and possibly hem in Russia's own operations, according to a European official and a senior US official. The administration's new plan, which was devised before the Russian buildup in Latakia, Syria, has not been co-ordinated with Russia, an administration official said, and the US made it clear last week that its campaign against Islamic State would not be thrown off course by the Russian military strikes.

But it seems likely that an effort will be made to “deconflict” US and allied air operations in northern and eastern Syria from Russia’s air strikes once the new operation is underway. Pentagon officials held a one-hour teleconference with their Russian counterparts on Thursday and presented a proposal for ways to minimise the risk of unintended confrontation. A follow-up discussion has yet to be held.

In addition, the US and Turkey continue detailed planning to use Arab militias to close a 60-mile expanse of border from the Euphrates River west to Kilis. The two countries reached agreement in late July on the basic concept, but now detailed planning is going forward on the assumption that Obama and President Recep Tayyip Erdogan of Turkey will bless it when it is done.

New York Times