Libya says British warplanes have hit a major oil field, damaging a pipeline connecting the field to a Mediterranean port.
Libyan Deputy Foreign Minister Khaled Kaim said three guards were killed in the reported strike on the Sarir oil field. British officials had no immediate comment on Kaim's claims.
Britain is part of the NATO force enforcing a UN-mandated no-fly zone over Libya.
Earlier, forces loyal to Muammar Qaddafi used tanks and heavy artillery to target rebel positions in the western city of Misurata.
Meanwhile, French Foreign Minister Alain Juppe said NATO air strikes in Libya risked getting "bogged down."
Juppe said pro-Qaddafi forces were making it harder for alliance pilots to distinguish them from civilians by hunkering down in populated areas.
In Washington, U.S. Secretary of State Hillary Clinton said Qaddafi "knows what he must do."
"There needs to be a cease-fire," she said. "His forces need to withdraw from the cities that they have forcibly taken at great violence and human cost. There needs to be a decision made about his departure from power and his departure from Libya."
Qaddafi is reported to have sent a letter to President Barack Obama, urging the U.S. leader to end what Qaddafi called "an unjust war."