The US-led coalition fighting the Islamic State group is “intensifying” its campaign in northern Syria, President Barack Obama says.
Speaking at the Pentagon, Mr Obama said the US would not send additional troops to Syria but would increase support for the country’s moderate opposition.
He added that “an effective partner on the ground” was needed to defeat IS.
It comes as IS militants were reported to have regained control of the town Ain Issa from Kurdish-led forces.
Mr Obama’s remarks followed some of the heaviest bombing of IS since coalition strikes began in September last year.
The bombing targeted Raqqa, IS’s de facto capital in Syria.
“We’re intensifying our efforts against Isil’s base in Syria,” said Mr Obama, using another name for the militant group.
“Our air strikes will continue to target the oil and gas facilities that fund so much of their operations.
“When we have an effective partner on the ground, Isil can be pushed back,” he added.
However, he warned that the campaign against the group would “not be quick”.
Earlier on Monday, IS militants reportedly regained control of a town near their northern Syrian stronghold of Raqqa from Kurdish-led forces.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said jihadists overran Ain Issa – 50km (30 miles) from Raqqa – and nearby villages.
But a spokesman for the Kurdish Popular Protection Units (YPG), which seized the town from IS two weeks ago, said it was still resisting the IS assault.
YPG fighters and allied Syrian rebels had only seized Ain Issa from IS two weeks ago.
The jihadist group had suffered a series of defeats in areas along the Turkish border, since being forced to withdraw from the town of Kobane in January.
However, in recent weeks, IS has launched several deadly counter-attacks against the Kurds.