US warns Iran on backing Houthis as Tehran calls for end of airstrikes

Washington warned Thursday it would not stand by while Tehran supports the Houthis in Yemen, as Iran's supreme leader denounced Saudi-led air strikes in the country as "criminal acts".

In the most direct American criticism yet of Tehran's backing for the Houthis, Secretary of State John Kerry said Washington would not accept foreign interference in Yemen.

"There have been - there are, obviously - flights coming from Iran. Every single week there are flights from Iran and we've traced it and know this," Kerry told PBS television.

"Iran needs to recognise that the United States is not going to stand by while the region is destabilised or while people engage in overt warfare across lines, international boundaries in other countries."

Washington has backed the Saudi-led air campaign which began last month as Houthis advanced on Yemen's main southern city of Aden after seizing Sanaa.

Although Iran and the Houthis have established diplomatic ties in recent weeks and Iranian politicians have spoken out in favour of the Houthis, the extent and nature of Iran's support for the militia remains unclear. 

On Thursday, Iran's leader Ayatollah Ali Khamenei warned that the campaign against the Houthis must end.

"This move is not acceptable in the region and I would warn that they must stop these criminal acts in Yemen," he said on his website. 

President Hassan Rouhani, in a speech in Tehran on Thursday, called for a ceasefire in Yemen to allow talks to resolve the fighting, AP reported.