Iraqi government forces have slowed their advance on the major city of Fallujah, held by Islamic State (IS) fighters, as U.S.-backed Kurdish and Arab fighters opened a major new offensive against the extremist militants in northern Syria.
At the United Nations, meanwhile, Britain called for an emergency meeting of the Security Council to push forward with air drops of humanitarian aid to areas besieged by government forces.
In Syria, the push by the Syrian Democratic Forces toward Manbij, a town near the Turkish border, is the latest effort by U.S.-backed forces to squeeze the extremist group and uproot it from its self-declared capital, Raqqa.
Officials said taking Manbij would deny IS militants the ability to move supplies and foreign fighters across the border from Turkey.
The region is the last remaining tract of territory IS holds at the border with Turkey, a top priority for the U.S.-led campaign.
The Syrian Observatory for Human Rights said on June 1 that the Kurdish and Syrian Arab troops seized control of 16 villages and were about 15 kilometers from Manbij.
Aircraft from the U.S.-led international coalition pounded Manbij before dawn, killing at least 15 civilians, the British-based monitoring group said.
The clashes were "fierce and intense," said Nasser Haj Mansour, an adviser to the Syrian Democratic Forces.
One unnamed Kurdish official told Reuters on June 1 that IS defenses on the west bank of the Euphrates River had collapsed at the start of the campaign earlier this week, but it was too soon to say how the Manbij battle would go.
Unnamed U.S. officials were quoted as saying thousands of forces were deployed in the operation, which could take weeks. A small number of U.S. Special Operations Forces will support the push on the ground, acting as advisers, they said.
At the United Nations, meanwhile, France and Britain urged the world body to begin humanitarian aid airdrops in Syria.
The call for action came despite land deliveries of aid supplies to two towns besieged by Syrian government forces where civilians are facing food shortages.
Britain called the emergency Security Council meeting for June 3 to discuss humanitarian access and to press ahead with the airdrops agreed to at a meeting last month of the International Syria Support Group.
The 20-country support group set June 1 as the deadline for aid convoys to reach all besieged areas.
In Washington, U.S. State Department spokesman John Kirby said the United States also supported moving forward on plans for the airdrops.
According to the United Nations, about 592,000 people live under siege in Syria -- the majority besieged by regime forces -- and another 4 million live in hard-to-reach areas.
Peace talks to end Syria's five-year war stalled in April after escalating fighting on the ground and lack of humanitarian aid prompted the opposition to walk out.
Islamic State militants also faced an offensive in the Iraqi city of Fallujah, where Prime Minister Haidar al-Abadi ordered his troops on June 1 to slow an advance at the city gates, two days after the army poured into rural areas on the city's outskirts.
The order was intended to limit harm to tens of thousands of civilians trapped in the city.
Fallujah has been a bastion for the IS militants, who raised their flag in the city in 2014 before sweeping through Iraq's north and west.
The United Nations has warned that militants are holding hundreds of families in the center as human shields.