But the Russian Prime Minister, Vladimir Putin, told him that the situation was an internal matter for Russia and should not affect relations with the US.

The two men met for about an hour in Oslo, where they had taken part in a ceremony to honour the murdered Israeli leader Yitzhak Rabin.

The BBC's Lindsay Marnoch: "Tank after tank after tank"

Mr Clinton highlighted the humanitarian and political costs Moscow could face if it pursues its strategy in Chechnya, a US official said.

Mr Putin defended Russia's campaign, which Moscow says is aimed at Islamic militants responsible for a spate of bomb attacks across Russia and incursions into the republic of Dagestan.

Russian soldiers fire a howitzer from the positions in the outskirts of Gudermes

At the Rabin ceremony, Mr Putin called for the international community to help Russia fight rebels using "terror" in the North Caucasus.

However, he said Moscow was "ready to receive representatives from international organisations" to examine the humanitarian problem.

Following earlier talks in Oslo, Russian Foreign Minister Igor Ivanov gave permission for a delegation from the Organisation for Security and Co-operation in Europe to travel to Chechnya and Dagestan.

A Norwegian-led team will travel to the region by the end of next week, a Norwegian foreign ministry spokesman said.

Chaos at border

Thousands of refugees are stuck inside Chechnya's border with the Russian republic of Ingushetia, where they are hoping to escape Russia's continuing artillery and aircraft bombardment.

Few have been allowed to leave despite assurances from Russia that the border is open. One of the few people to cross on Tuesday said a line of vehicles 20km long was waiting to leave Chechnya.

Only 100 to 150 people were allowed to cross on Tuesday, and 150 people on Monday.

Reports from the scene said desperate men and women pressed against barbed wire barriers as they tried to pass through the tiny checkpoint. Russian media reported that at least four people had died amid scenes of chaos.

A BBC correspondent on the border, Andrew Harding, said the people were hemmed in like cattle.

Closing in on Grozny

Inside Chechnya, increasing numbers of people are trying to flee the capital Grozny.

Russian warplanes and artillery again bombarded the city on Tuesday morning, hitting the northern outskirts as well as the airport, Chechen officials said.

The districts of Staropromyslovski, Katayama, Tridsachestoi Uchastok and Ivanovo in the northwest came under fire, as well as the disused Sheikh Mansur airport in the north, the French news agency AFP quoted a spokesman of the Chechen armed forces as saying.

Russian soldiers make their way to positions in the Naurski region.

Overnight, the Russian airforce hit the western Chechen rebel stronghold of Bamut and Urus-Martan in the same region, the spokesman said.

The eastern town of Gudermes was also hit by bombs and artillery shells, the Russian Defence Ministry said.

Russian forces are reported to have taken up positions 4km (2.5 miles) from Grozny.