UN officers said fierce shelling erupted in the morning along the eastern front lines as Rwanda Patriotic Front (RPF) rebels resumed an offensive on government forces holed up at the airport.
'There is shelling everywhere - very heavy mortar and artillery fire. It was impossible for our plane to land,' said the director of the UN Assistance Mission in Rwanda (Unamir), Abdul Kabia.
Mr Kabia said the situation was tense and at least one shell hit the airport tarmac, but there were no casualties. A Canadian transport plane carrying UN troops and journalists was forced to return to Nairobi without landing at Kigali.
The airport is held jointly by Unamir and government troops, who have refused to withdraw.
The bombardments concentrated on RPF positions in a valley on the eastern outskirts of Kigali, the presidential palace and Kanombe camp, a fortified base for elite government troops.
More than 200,000 people are estimated to have been killed in massacres, mostly of Tutsis by Hutu troops and militiamen, across the central African state since the killing of the Hutu president, Juvenal Habyarimana, and the Burundian president in a mysterious rocket attack on their plane at Kigali airport on 6 April.
Rebels have advanced on the airport since last Wednesday. Kigali is the only air link with the outside world for the 270 UN troops and relief agencies in Rwanda. Only one UN flight has landed there since Thursday.
Asked whether airport closures were making the position of the UN force untenable, Mr Kabia said: 'We don't plan to go yet. We took precautions to ensure the security of our people at the airport.
'Our activities are extremely restricted. We are moving around a bit but we have suspended most operations,' he added.
RPF leaders have ruled out a ceasefire until massacres stop and have refused to negotiate with the Hutu interim government, which the RPF has condemned as 'a clique of killers'. UN officials had said they believed that any ceasefire would be brief but had hoped it would allow peace negotiations to start.
The aid charity Oxfam on Sunday accused the UN Security Council of dithering while thousands died daily in the world's bloodiest example of genocide since the slaughter in Cambodia in the 1970s.Reuse content