Sri Lanka's government has refused to agree to UN Secretary General Kofi Annan visiting tsunami-hit north-eastern areas under rebel Tamil Tiger control.
Mr Annan says his itinerary in Sri Lanka was set by the government
Mr Annan toured parts of Sri Lanka's devastated coast, where more than 30,000 people died in December's tsunami.
"I would like to visit all the areas," Mr Annan said, but added that he was a guest and the government had set the itinerary.
Tensions between the rebels and the government are threatening to overshadow the aid effort.
More than 150,000 people have been killed across the Indian Ocean region.
The UN has warned the death toll may soar as the fate of many thousands is still unknown following the 26 December disaster.
During his trip to Sri Lanka, Mr Annan flew over battered coastal areas, including the southern town of Galle, where US Secretary of State Colin Powell visited a day earlier.
"From the air I saw a beautiful country, but there has been a lot of damage," Mr Annan told reporters.
He visited survivors in the tourist town of Hambantota, before flying up the east coast to the town of Trincomalee.
Tamil Tiger rebels were first to provide aid in many areas
In Hambantota, he was met by Prime Minister Mahinda Rajapakse and World Bank head James Wolfensohn.
On Friday, the Tigers said Mr Annan had accepted an invitation to visit their leader, Velupillai Prabhakaran.
The BBC's Dumeetha Luthra in Colombo says privately UN officials said Mr Annan was keen to go, but under government pressure was forced to refuse.
It appears the government was concerned a visit by the UN secretary general would legitimise the rebels' demands.
The Tigers say little aid has been sent to north-eastern areas of the country they control - claims denied by the government.
And the rebels have warned of serious consequences if government soldiers are not withdrawn from welfare camps accommodating Tamil civilians.
The commander of the Tamil Tiger rebels' naval wing has dismissed Sri Lankan government reports that 5,000 of his men and a major naval base were wiped out in the tsunami.
Special Commander Soosai said this and allegations that the Tiger leader and intelligence chief had been killed in the tsunami were false propaganda.
The BBC's Frances Harrison was allowed to visit the Vadduvahal headquarters of the Sea Tigers, as they are known, on the coast just north of Mullaitivu Town - one of few journalists ever allowed there, according to the rebels.
Our correspondent says there were still some signs of mud, but no massive destruction.
The Sea Tigers say they lost six men from their political wing, but not thousands, as alleged by the Sri Lankan government.
A ceasefire between the Tigers and the government has remained largely intact, even though the Tigers pulled out of negotiations in April 2003, saying the government was not honouring pledges it had made.
The Tigers have been fighting for more than two decades for self-government in the north and east, which they consider the Tamil homeland. More than 60,000 people have been killed during the conflict.