A ceasefire between the United States and Iran appeared in jeopardy on Monday after the US said it had seized an Iranian cargo ship that tried to run its blockade and Tehran vowed to retaliate, refusing for now to join new peace talks, Reuters reports.

A senior Iranian source told Reuters the continuation of the US blockade was undermining the prospect of peace talks, and Tehran's "defensive capabilities", including its missile program, were not open to negotiation.

And a Pakistani security source said Pakistan's key mediator, Field Marshal Asim Munir, had told US President Donald Trump the blockade was an obstacle to talks, and that Trump had replied that he would consider the advice.