US Homeland Security Secretary Michael Chertoff says officials are increasing security at airports, train and bus stations but so far have no intelligence linking recent terrorist activity in Britain to threats against the United States.
Chertoff said in a television interview on Sunday that he is worried about so-called terrorist "copy cats" who may try to imitate the vehicle bomb plots.
Chertoff urged Americans to remain alert to possible threats, suspicious packages and activities.
However, he said US intelligence agencies have received no credible evidence of a pending attack in the United States, and said the overall US threat level is not being raised.
At the same time, speaking in another television interview Sunday (CNN's Late Edition), Chertoff said U S authorities are concerned about the threat of terrorism in the coming months.
Britain raised its terror alert level to the highest status of "critical" Saturday after incidents in Glasgow and London.