The United States is demanding answers from Nigeria over the whereabouts of Liberia's ex-leader after he went missing from his exiled home on Monday.
Charles Taylor is facing 17 counts of war crimes
Secretary of State Condoleezza Rice said reports of Charles Taylor's escape were a matter of "utmost seriousness".
US President George W Bush is facing calls to cancel talks with his Nigerian counterpart in Washington on Wednesday.
The chief prosecutor of the UN-backed war crimes court condemned the reported escape of the alleged war criminal.
Desmond de Silva of the special tribunal in Sierra Leone described it as "an affront to justice".
United Nations Secretary General Kofi Annan has said he is deeply concerned by reports that Mr Taylor has gone missing.
He called for countries in the region not to give refuge to Mr Taylor and to comply with a request to hand him over.
The news of Mr Taylor's disappearance from a villa where he lived came just 48 hours after Nigeria had said that Liberia was free to "take Taylor into custody".
Mr Taylor was indicted on 17 charges of war crimes and crimes against humanity, concerning his alleged backing for Sierra Leone's rebels, shortly before stepping down in 2003.
He resigned under a deal to end the Liberian civil war, which he started in 1989, going into exile in Calabar, in south-eastern Nigeria.
White House spokesman Scott McLellan refused to say whether the planned meeting between Nigerian President Olusegun Obasanjo and Mr Bush would go ahead.
1989: Launches rebellion
1991: RUF rebellion starts in Sierra Leone
1995: Peace deal signed
1997: Elected president
1999: Lurd starts rebellion to oust Taylor
June 2003: Arrest warrant issued
August 2003: Steps down, goes into exile in Nigeria
Mr McLellan said Washington was "looking for answers from the Nigerian government about the whereabouts of Charles Taylor".
He added that it was the responsibility of the Nigerian government to see that Mr Taylor was handed over to the tribunal in Sierra Leone.
Ms Rice, meanwhile, told a congressional panel that Washington would take the matter very seriously "if indeed he (Mr Taylor) had fled."
She stressed that Nigeria had made a commitment that Mr Taylor would be monitored while in exile.
Ms Rice did not respond to a suggestion by one of the senators that it would be inappropriate to hold the Bush-Obasanjo talks under the circumstances.
Mr Taylor's whereabouts are not clear but his spiritual advisor Kilari Anand Paul told the BBC News website that Mr Taylor was in the Liberian bush, from where he first launched his rebellion.
He also said the former Liberian leader would be happy to face justice in The Hague but not in Sierra Leone.
Nigeria's President Olusegun Obasanjo is "very shocked" by Mr Taylor's disappearance on Monday, Information Minister Frank Nweke told the BBC's Focus on Africa programme.
Nigeria has set up a panel to investigate the matter, and to establish whether he escaped or was abducted, Mr Nweke said.
Lobby group Human Rights Watch says Mr Taylor's disappearance is a "disgrace".