The US secretary of state, Hillary Clinton, has said time is running out for diplomacy over Iran's nuclear programme and confirmed talks aimed at preventing Tehran from acquiring a nuclear weapon would resume in mid-April.

With speculation over a possible US or Israeli strike adding urgency to the next round of discussions in Istanbul, set for 13 April, Clinton said Iran's "window of opportunity" for a peaceful resolution "will not remain open forever".

She also expressed doubt over whether Iran had any intention of negotiating a solution that satisfies the US, Israel and other countries that believe Tehran is trying to develop nuclear weapons. Tehran contends the programme is solely for peaceful energy and research purposes.

"We're going in with one intention: to resolve the international community's concerns about Iran's nuclear program," Clinton said after attending a security conference in Saudi Arabia. "Our policy is one of prevention, not containment. We are determined to prevent Iran from obtaining a nuclear weapon.

"We enter into these talks with a sober perspective about Iran's intentions. It is incumbent upon Iran to demonstrate by its actions that it is a willing partner and to participate in these negotiations with an effort to obtain concrete results."

Clinton's remarks followed President Barack Obama's announcement on Friday that the US was moving ahead with penalties aimed at depriving Iran of oil revenue, while also working with Saudi Arabia and other Persian Gulf states to safeguard global petroleum supplies.

Clinton also urged Gulf governments to develop a co-ordinated defence strategy against Iranian missiles. With tensions rising in the region, she said American and Gulf militaries should cooperate to improve maritime security as well.

She also said the US would co-operate with the region's Sunni governments to counter Iranian threats against shipping in the Gulf and Tehran's support for "the Assad regime's murderous campaign".

Before heading to Turkey for a 60-nation "Friends of the Syrian People" meeting on Sunday, Clinton condemned the Assad government for shelling civilian neighbourhoods and targeting mosques and churches.

She said these attacks have continued despite Assad's acceptance of UN mediator Kofi Annan's plan to end the crisis. That plan includes an immediate ceasefire and an eventual democratic transition.