The White House yesterday accused the President of Yugoslavia, Mr Slobodan Milosevic, of breaking his promise on Kosovo refugees and said the US and its allies were working on options for action in the crisis.
"The humanitarian situation is becoming of increasing concern to us," a White House spokesman, Mr P.J. Crowley, said.
"President Milosevic pledged late last week to allow greater access for humanitarian workers to address the situation with respect to displaced. So far, his words are not backed up by action," he said.
A senior US diplomat warned yesterday that Kosovo is facing a "humanitarian nightmare".
"If we don't get people back to their homes in the next week or two, if we don't start that process, we are really going to have a humanitarian nightmare," Mr Christopher Hill said during a visit to Orahovac, a town in south-west Kosovo, abandoned by a population fleeing violence.
Mr Hill, who is the US ambassador to Macedonia, has been entrusted with the task of examining the Kosovo crisis on behalf of the Clinton administration. He was visiting Orahovac to assess conditions for returning people, and said there was considerable damage and homes had been recently burned.
The town's mayor, Mr Andjelko Kolasinac, said yesterday that 2,300 people had returned in the space of a week.
Serbian security forces kept up their operations against ethnic Albanian guerrillas in central Kosovo yesterday, ignoring pleas from Western powers for a ceasefire.
One UN agency said the Serbian attacks were depopulating the province. Another more than doubled - to 70,000 - its estimate of the number of people displaced by fighting over the past week.
The official Yugoslav news agency Tanjug reported that "terrorists" - its term for the KLA - had been "neutralised" at Lausa, west of the provincial capital Pristina.
Western powers on Monday stepped up the pressure on Serbia to halt its offensive, with the US warning that NATO was fine-tuning plans for the crisis which could be ready "very, very quickly".
The US State Department spokesman, Mr James Rubin, declined to say what NATO's plans were, but NATO foreign ministers in late May asked military experts to look at possible preventive deployments in Albania and Macedonia, which border Kosovo.
Mr Rubin had said that NATO had already approved the plans, but diplomats in Brussels said they were still being worked on.
The UN World Food Programme in Rome said as many as 70,000 people had fled the fighting over the past week and that many were running out of food and water. Estimates of the number of people displaced within the province and fleeing abroad since clashes began in February are now of the order of 180,000.
In Geneva, a spokesman for the UN refugee agency (UNHCR), Mr Kris Janowski, stopped short of accusing Serbian forces of ethnic cleansing but said the fighting was having the same effect.
Russia, a traditional Serb ally which opposes foreign military intervention in Kosovo, is to send its deputy foreign minister to Belgrade today to step up diplomatic efforts, the foreign ministry said.
Mr Nikolai Afanasyevsky, a regular visitor to Belgrade in recent months, also plans to visit Pristina for talks with ethnic Albanian officials.
Belgrade media reported, meanwhile, that the new drive by Serbian forces had brought a large area of central Kosovo under government control and made further inroads in the far west.
Reports from various sources said the new fighting had resulted in security forces taking control of a large area around Klima, a town between Pristina and Pec, the province's second largest town.
The Bosnian Serb news agency SRNA reported yesterday that fierce fighting was still taking place north of Klima.