Following days of speculation, US Brigadier-General Daniel Leaf told reporters that Nato aircraft had attacked two targets - one large and one small - last Wednesday.

He said it was possible that civilians had been injured in both incidents.

Media fears

As Nato's bombing of Yugoslavia continued on Monday, Western journalists were invited by the Serbian information minister to visit the headquarters of Serb TV station RTS in the centre of Belgrade.

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The move followed rumours that it might be targeted by Nato. One American television channel, which had a satellite facility there, pulled out all its equipment and many of the other Western organisations cancelled their satellite transmissions.

BBC World Affairs Editor John Simpson - who is in Belgrade - said several organisations, including the BBC, had decided not to take part in what would presumably have been a propaganda exercise.

Elsewhere, seven to eight "powerful explosions" were heard in the area of Nis, southeastern Serbia, on Monday at around 2300 local time (2100GMT), the Yugoslav state news agency Tanjug reported.

Earlier, a UK military officer said command post used by Serbian troops carrying out ethnic cleansing in Kosovo had been attacked by RAF jets.

US Apache helicopters are due to be deployed soon

Group Captain Glenn Edge said RAF planes also bombed a site used to store and repair surface to air missiles.

Meanwhile, the US has sent several hundred paratroopers to Albania to protect its Apache attack helicopters, which are due to be deployed shortly.

An army official said it was the largest contingent of US army troops to be sent from US bases since Nato air strikes against Yugoslavia began.

'Massive movements'

In Kosovo itself, hundreds of thousands of refugees have been seen heading towards Albania, Montenegro and Macedonia, the World Food Programme has said.

People were reportedly queuing for many kilometres to cross the Albanian border, which remained closed after Yugoslavia severed diplomatic relations with Tirana.

A UN spokesman said that Serbian forces were forcibly preventing people from leaving Kosovo, turning back trains and buses.

The Yugoslav army is also reported to be expelling residents of three predominately Muslim villages near the Montenegrin border with Kosovo.

More than 400 people from the villages arrived in the town of Rozaje, bringing with them tales of expulsion similar to those told by Kosovo Albanians.

US President Bill Clinton has asked the US Congress for $6bn in emergency spending for the military operation and to increase aid to the Kosovo Albanians fleeing the province.

No diplomatic progress

On the diplomatic front, Mr Clinton and Russian President Boris Yeltsin had their first direct discussion on Kosovo since the Nato strikes started.

The two men spoke by phone, but failed to reconcile their differences over the handling of the crisis.

The UN Secretary-General, Kofi Annan, has said he will visit Moscow at the end of the month to try to help resolve the crisis.

Nato: Two incidents

Brigadier-General Leaf: "Nato may have inadvertantly struck civilian-type vehicles"

Nato's admission that it may have caused civilian casualties came several days after Belgrade accused the alliance of killing more than 70 Kosovo Albanian refugees near the town of Djakovica.

Nato said there had been two incidents, in different places and at different times.

In the first, it said a military target had been hit, northwest of the town of Djakovica - civilian casualties could not be explained.

The second target, southeast of Djakovica, had been a mixed convoy which pilots believed included military vehicles. Nato had already admitted this incident.

"This is a very complicated scenario and we will never be able to establish all the exact details," Gen Leaf said.

Asked about reports that British Harrier pilots had warned about the refugee column, Gen Leaf said: "This occurred at approximately the same time but they weren't in communication with the aircraft in question."

BBC correspondent Nick Childs in Brussels says the Nato account still does not explain some of the television pictures from the ground.

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