Iran should not doubt major powers' determination to stop it getting a nuclear bomb, British Foreign Secretary William Hague said on Thursday, but he also hinted that sanctions could be eased if Iran gave ground in a long-running nuclear dispute.

In a major foreign policy speech, Hague also warned Syrian President Bashar al-Assad and his allies, engaged in a bloody crackdown on a year-long revolt, that they faced more punitive sanctions, international isolation and possible prosecution unless they allowed a democratic transition to unfold.

Looking ahead to talks on Iran's nuclear programme, set to resume next month after a gap of more than a year, Hague said: We approach these talks with sincerity and a genuine desire for a breakthrough. This can only come if Iran enters the talks in a new spirit.

The Islamic Republic has said it expects to reopen talks with six major powers, including Britain, on April 13 and that Turkey has offered to host the meeting.

Iran and the West are locked in confrontation over its nuclear energy programme, which Tehran says is peaceful but Western powers suspect is aimed at developing an atomic bomb.

Assuring Tehran that the powers do not seek regime change, Hague said: We look to the Iranian government to prove to the world that their nuclear programme is for peaceful energy, not for nuclear weapons, and to give up any plans to acquire them.

Noting that Iran faced unprecedented sanctions, including a European Union oil import ban due to take effect on July 1, Hague appeared to hint that Iranian concessions could be rewarded with an easing of sanctions.

It is in the Iranian government's power to end this isolation, and if they negotiate seriously on the concerns over its nuclear programme we will respond, he said in the text of a speech to an annual Lord Mayor's banquet in London.

Russia has advocated a step-by-step plan in which sanctions would be eased in return for verifiable steps by Tehran to defuse concerns about its nuclear intentions.

RESOLVE

However, Hague said that if the Iranians do not seize this opportunity, they should not doubt our resolve to prevent nuclear proliferation in the Middle East.

Growing tensions over Iran's nuclear work have led to speculation that Israel or the United States could launch a pre-emptive military strike.

On Syria, Hague said Assad and his allies must be left in no doubt that if there is not a political transition that reflects the will of the Syrian people, then they will be shunned by the international community and we will close every door to them. They will face still more sanctions.

And they will be pursued by mechanisms of justice, he said, calling the behaviour of Assad's government futile and morally indefensible.

United Nations' officials have compiled a list of Syrian figures suspected of crimes against humanity during attempts to suppress an uprising in which, according to U.N. figures, government forces have killed 9,000 people.

But opposition from Russia and China means the accused are unlikely to appear in the dock at the international war crimes court any time soon.

Hague said he expects a Friends of Syria meeting in Istanbul on Sunday to adopt new measures to increase pressure on Assad, support Syria's opposition and boost the mission of Kofi Annan, the special U.N. and Arab League envoy on Syria.

Earlier on Thursday, Britain said it would double non-military aid to Assad's opponents and expand the scope of the assistance offered, possibly including secure telephones to help activists communicate more easily.

Hague said it was in Britain's vital national interest to help Arab Spring democracy movements to succeed, because of the democratic, economic and security benefits they would bring. And he said Britain would engage with new political parties in the region, including those drawing their inspiration from Islam.

The Arab Spring has led to a sharp rise in the influence of Islamists in several North African countries.

(Editing by)