Mortar rounds flew indiscriminately in Monrovia throughout the day -- with at least two rounds hitting the US embassy -- as rebels with the Liberians United for Democracy and Reconciliation (LURD) battled supporters of President Charles Taylor for control of the capital.

US President George W. Bush said he was "monitoring" the crisis, but gave no indication when he would commit US forces to support a West African peacekeeping operation.

The Pentagon however announced that 4,500 sailors and Marines from the Horn of Africa and the Red Sea are being sent to the Mediterranean, where they could sail to Liberia in as little as seven to 10 days.

Defense Secretary Donald Rumsfeld signed the deployment orders on Saturday, Pentagon spokesman Lieutenant Dan Hetlage said.

Earlier in the day 41 soldiers from a US naval "anti-terrorism security team" based in Rota, Spain, flew into Monrovia to help guard the embassy.

And 100 military personnel, three helicopters and an MC-130 transport plane are in position in nearby Senegal and Sierra Leone in case a decision is made to evacuate the Monrovia embassy, officials said.

Meanwhile, UN Secretary General Kofi Annan asked Washington to decide whether it was going to send troops to Liberia.

Annan "urges the United States to spare no effort to support this deployment and to announce its own decision on the deployment of US troops before it is too late," said his spokesman, Fred Eckhard.

Bush said earlier that US troops could be deployed if Taylor steps down, and following the deployment of African peacekeeping troops.

Annan also renewed his call to the Economic Community of West African States -- which has committed to send troops -- "to deploy without delay the proposed vanguard force to restore calm and security."

In Washington, US officials condemned what it called the rebel "reckless and indiscriminate" shelling of Monrovia, and called for a halt to the offensive.

US diplomats contacted LURD representatives in Guinea and Ghana to urge an end to the assault, a State Department official told AFP on condition of anonymity.

Deputy State Department spokesman Philip Reeker said Washington is "strongly condemning" the LURD for the attack, which hurts civilians as well as humanitarian workers in the city.

"This breaking of the ceasefire is something we call on them to end," Reeker told reporters.

If Washington is to trust the LURD "to participate in the democratic governance of Liberia, we need to be able to see them keep their commitments now," he added.

Reeker also called on Liberia's neighbors to stop any movement of arms or troops across their borders, specifically naming Guinea.

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