YANGON (AFP) - The United Nations' special envoy for Myanmar is to embark on an Asian tour to step up diplomatic efforts to tackle the crisis, as the death toll from the junta's crackdown on dissent passed 600 on Friday (April 9).

The push by Ms Christine Schraner Burgener comes amid mounting international concern over events in Myanmar, which has been rocked by daily protests since the military ousted civilian leader Aung San Suu Kyi and seized power on Feb 1.

Ms Burgener will start her trip in Thailand and will also visit China, though exact details and timings for her trip have not been confirmed.

At least 614 civilians have been killed in the military's crackdown on protests and nearly 3,000 have been arrested, according to the Assistance Association for Political Prisoners, a local monitoring group.

There was more bloodshed early on Friday, with rescue workers reporting that at least four people were killed when security forces broke up protest barricades in the city of Bago, 65km northeast of Yangon.

UN rights officials said the military is making increasing use of heavy weaponry including rocket-propelled and fragmentation grenades, heavy machine guns and snipers.

The violent response has drawn intense international criticism and, on Thursday, Washington imposed another set of sanctions, this time on Myanmar's state gem company, as it seeks to deprive the junta of sources of income.

Call for unity

UN envoy Burgener wants face-to-face meetings with the generals, but she has not received permission to visit Myanmar.

"She, of course, stands ready to resume dialogue with the military to contribute to a return to Myanmar's democratic path, peace and stability," UN spokesman Stephane Dujarric said.

Other than China - a key player, with historical links to the military, that has more recently cultivated close relations with Ms Suu Kyi - Ms Burgener also hopes to visit member countries of regional bloc Asean.

"As she has highlighted repeatedly, a robust international response to the ongoing crisis in Myanmar requires a unified regional effort involving neighbouring countries who can leverage influence towards stability," Mr Dujarric said.

An Asean summit on Myanmar is scheduled for the end of the month, but diplomats say the bloc is deeply divided over the crisis.

"At one end, there are Thailand, Laos and Cambodia, who are in the mode of 'back off, there's nothing to see, it's a question of internal politics'," one diplomat said, while Singapore, Malaysia and Indonesia are open to a more active role for Asean.

The UN Security Council is set to meet informally on Friday to hear from Myanmar lawmaker Zin Mar Aung on behalf of the Committee for Representing Pyidaungsu Hluttaw (CRPH), which represents the deposed civilian government.

Earlier this week, the CRPH, which says it has amassed nearly 300,000 pieces of evidence showing rights abuses by the junta, began talks with the UN's Independent Investigative Mechanism for Myanmar.

The military has defended seizing power, pointing to what it said was fraud in November's election, which Ms Suu Kyi's National League for Democracy won comfortably.

Journalists arrested

When it took over, the military imposed a one-year state of emergency, but spokesman Zaw Min Tun told CNN this may be extended by up to a year, though he insisted elections would then be held.

The junta has steadily choked off communications, throttling mobile data and imposing nightly Internet shutdowns, while raiding newsrooms and arresting journalists.

Local news outlet Mizzima reported on Thursday that its co-founder Thin Thin Aung and another former employee have been detained.

"Mizzima is concerned for the danger to their lives and calls for their immediate release," it said on its Facebook page.

Mizzima had its broadcasting licence revoked last month and its journalists have been working underground ever since.

Since Feb 1, nearly 60 media workers have been detained or charged while covering the coup, according to Reporters Without Borders.