United States President Barack Obama says that time is running out to produce a diplomatic solution to the Iranian nuclear issue and that “the window for solving this issue diplomatically is shrinking.”

In a joint news conference with British Prime Minister David Cameron at the White House on Wednesday, local time, the U.S. president said that Washington conveyed the opinions to Iranian leaders that Tehran must be sincere and serious about nuclear negotiations.

In addition, Obama and Cameron reaffirmed a plan to transfer security authority to Afghanistan through the withdrawal of U.S. and NATO forces, despite a recent killing of Afghan civilians by a U.S. soldier.

The two leaders also called for an end to the bloody crackdown on antigovernment protesters in Syria, but added that now is not the time to consider military intervention in Syria.