US Defence Secretary Ashton Carter on Tuesday visited the Turkish air base near Syria that has become a key hub for American-led air raids against Islamic State jihadists, urging Ankara to play a more active role in the coalition.

Turkey earlier this year allowed US forces to use the Incirlik base outside the southern city of Adana for bombing raids against IS targets in Syria.

The Turkish air force later also joined the campaign, making a handful of officially-announced air strikes, although it is unclear so far what impact its involvement has made.

"Turkey has an enormous role to play and... we appreciate what they're doing. We want them to do more," Carter told reporters while travelling to Incirlik, in a statement released by the Pentagon.

He said Turkey could do even more beyond its role of hosting the coalition forces at Incirlik and should also ensure full control of the border with Syria.

"We want Turkish forces to join in the air and the ground and as appropriate," Carter said, without giving further details.

He also said Washington wanted to see Gulf counties in the coalition -- which include Saudi Arabia and Qatar -- to do more.

"As in the case of all the coalition partners, we're giving them some specific ideas."

A US diplomatic source said the main goal of Carter's visit was to meet US soldiers to thank them for their contribution to the air campaign. There was no plan for Carter to visit Ankara for political talks.

Gaining the use of Incirlik, which lies just 160 kilometres (100 miles) from the Syrian border, was a huge boost to the US campaign against IS, which had relied on air bases at a far greater distance.

The US source said Incirlik had now become a "major allied hub of activity".

"Together over the past month, we have achieved the highest operational tempo against ISIL since the start of the air campaign in August 2014," said the source, using a different acronym for IS.

IS "is no longer advancing on the battlefield and that is due in large part to the coalition's comprehensive military campaign that pressures them from the air and on the ground."

President Barack Obama said on Monday the US and its allies were hitting IS "harder than ever" and warned the extremists' leaders: "You are next."

Turkey's Western allies had in the past accused Ankara of not doing enough to fight IS and even on occasion collaborating with the group, allegations Turkish officials vehemently denied.