Syrian opposition leaders accused the Syrian government of essentially collaborating with the Islamic State group.

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ISTANBUL — Building on recent gains in Iraq and Syria, Islamic State group militants are marching across northern Syria toward Aleppo, Syria’s largest city, helped along, its opponents say, by the forces of President Bashar Assad.

In the countryside northeast of Aleppo on Tuesday, Islamic State group fighters fought rival Syrian insurgents amid fears that the Islamic State group was positioning itself to make Aleppo its next big prize.

Syrian opposition leaders accused the Syrian government of essentially collaborating with the Islamic State group, leaving the militants unmolested as they pressed a surprise offensive against other insurgent groups — even though the government and the Islamic State group are nominal enemies — and instead striking the rival insurgents.

At the same time, the rebels complained that the United States has refrained from contributing air support to help them fend off simultaneous attacks by the government and the Islamic State.

The United States has resisted calls for increased assistance to the rebel coalition, as it is a muddle of groups including, most notably, the al-Qaida-linked Nusra Front, even though the United States is seeking to recruit some of those insurgents to help it battle the Islamic State group.

The charges and countercharges of subterfuge and double-dealing underscored the complexity of the battlefield in Syria’s multifaceted war and the challenges it poses for U.S. policy.

Western officials have sought to play down the significance of the militant group’s recent gains, including Palmyra, the strategically placed World Heritage site in Syria, and Ramadi, the capital of Iraq’s Anbar province. But the fall of Aleppo would be a critical blow.

At stake is the survival of one of the few pockets of insurgent-held territory not dominated by the Nusra Front or the Islamic State group.

Northeast of the city of Aleppo, which is divided between government and rebel control, the main insurgent groups are Islamist factions not affiliated with the two most extreme groups.

Neither U.S. officials nor Syrian insurgents have provided proof of such direct coordination, although it has long been alleged by the insurgents.

What is clear is that Assad and the Islamic State group reap benefits by eliminating or weakening other insurgent groups. Assad can claim he is the only alternative to the Islamic State group, and the Islamic State group can claim it carries the banner of oppressed Syrians and Iraqis.