The UN has for the past three months focused on helping the South-East Asian nation recover from Cyclone Nargis which left nearly 140,000 people dead or missing.

But on Thursday several members of the UN Security Council warned Myanmar, formerly known as Burma, that it could face increased pressure if it did not move to release political prisoners.

They urged the military government to co-operate with Ibrahim Gambari, the UN special envoy to the country, to come up with a plan to release the prisoners.

Khalilzad said Myanmar was "misguided" if it thought it could buy time by allowing Gambari's visit.

"If there is not progress on these issues ... we would have to look at other measures, bringing more pressure to bear on the regime," he said without elaborating on the measures.

"Absent political progress, we see the potential for increased political instability and the council cannot remain indifferent to that," he said.

He said the council expected Myanmar's ruling generals to take advantage of Gambari's visit in mid-August to show progress.

Gambari last visited Myanmar in March to try to bring about reconciliation between the military government and its pro-democracy opponents.

On Wednesday, Ban Ki-moon, the UN secretary-general, called for strong co-operation from Myanmar after convening a meeting of the so-called Group of Friends to discuss Gambari's upcoming visit.

Sanctions renewed

Rights groups say Myanmar gems enter the US via third-party countries
Meanwhile in Washington, the US congress voted to renew a law that bans all imports from Myanmar, legislators said.

The Burmese Freedom and Democracy Act passed on Thursday renews a series of sanctions imposed since 2003 over the suppression of Myanmar's democracy movement.

The law maintains sanctions on the ruling generals until changes are made including steps towards reconciliation and democratisation, an end to attacks on ethnic minorities and the release of all "prisoners of conscience".

Earlier this week the US congress cleared another legislation aimed at keeping Myanmar's gems, including jade and rubies, from entering US markets via third-party countries.

Rights groups say that despite the long-standing ban on all imports, gems from Myanmar have been entering the US via Thailand, China, Taiwan, Malaysia and Singapore.