The UN special envoy to Burma has arrived in Rangoon in an effort to secure the release of detained opposition leader Aung San Suu Kyi.
Speaking before his arrival, Razali Ismail said he believed he had a "realistic chance" of seeing her during his visit.
Aung San Suu Kyi was taken into what military authorities described as "protective custody" after violence last Friday, which the military junta said left four people dead and 50 injured.
AUNG SAN SUU KYI
1990: NLD wins general election while Suu Kyi under house arrest; military does not recognise the result
1991: Wins Nobel peace prize
1995: Released from house arrest, but movements restricted
2000-02: Second period of house arrest
May 2003: Detained after clash between NLD and government forces
But officials from the United States embassy in Rangoon, who have visited the scene of the violence, said it appeared to have been a deliberate attack by "government-affiliated thugs".
BBC Burma analyst Larry Jagan says Mr Razali is facing his hardest task yet.
He will be telling the Burmese generals that they must immediately release Aung San Suu Kyi and resume dialogue or face the wrath of the international community, our correspondent adds.
UN officials have told the BBC that if Mr Razali is not given access to the opposition leader early on the visit he is likely to cut the trip short.
Aung San Suu Kyi was detained after clashes between her supporters and pro-government groups in the north of the country.
Some reports say more than 60 people died in the incident. Some eyewitnesses are in hiding, fearing military reprisals.
Aung San Suu Kyi is believed to have suffered cuts to her face and shoulder after the window of her car was smashed.
The military authorities said Aung San Suu Kyi was not injured, but no one has been allowed to see her.
Officials from the US embassy in Rangoon have visited the scene of the violence and found evidence of a "premeditated ambush," the US State Department said on Thursday.
It said the clash appeared to have been planned by "government-affiliated thugs" and called on Burma to "provide a full accounting of the dead, injured, and missing".
Washington also called Aung San Suu Kyi's detention "outrageous and unacceptable".
Razali wants Burma's generals and the opposition to resume talk
"If Ambassador Razali is not able to meet privately with Aung San Suu Kyi, the US will need to evaluate the utility of this UN-sponsored effort to support national reconciliation in Burma," the US statement warned.
Mr Razali helped broker peace talks between Aung San Suu Kyi and the military more than two years ago.
It led to Aung San Suu Kyi being released last May after nearly 20 months under house arrest, but progress has since stalled.
Since her release Aung San Suu Kyi has attracted large crowds during visits to her supporters around the country.
Under her leadership, the NLD won 1990 elections by a landslide but the military junta refused to hand over power.