Obama “expressed his appreciation that British air forces have joined with ours and those of other coalition members in striking ISIL in Syria as well as in Iraq,” the White House said in a statement.

British warplanes carried out their first airstrikes against Daesh-held oilfields in Syria within hours of parliament authorizing the action late Wednesday.

During their call, Obama and Cameron discussed additional steps coalition partners can take to set back and ultimately destroy Daesh, according to the White House.

The two leaders also “reiterated that all countries are welcome to join the existing coalition, if their political and military objectives in Syria are consistent with those of the coalition”.

That comes as Russia continues to carry out an independent air campaign in Syria, which it steadfastly maintains is aimed at Daesh and other terrorist groups, but which the U.S.-led coalition has said is targeting foes of Moscow’s Syrian ally, President Bashar al-Assad.

“They also discussed ways to further bolster our already robust counterterrorism cooperation, and the president, on behalf of the American people, accepted the prime minister’s condolences for the loss of life in the San Bernardino shootings this week,” said the White House.

At least 14 people were killed in a mass shooting that officials are now investigating as a terrorist plot following mounting evidence on social media that indicates the alleged perpetrators were radicalized.