US marines reportedly fired artillery shells at suspected rebel targets in the western Iraqi city of Fallujah this evening.
Witnesses said that at least seven artillery rounds were fired as US marines stepped up pressure on guerrillas ahead of a widely predicted assault on Islamic militants.
Earlier, US forces sealed off the rebel-held city, amid speculation an assault was imminent.
All roads in and out of the city are reported to have been closed.
The United Nations Secretary-General, Kofi Annan, has warned the US, Britain and the interim government in Iraq that an assault on Fallujah could further anger Iraqis and undermine elections planned for January.
Adnan Ali, an Iraqi government spokesman, said any civilian casualties in the offensive would be the responsibility of insurgents.
In a separate development, an internet statement attributed to the group of Abu Musab al-Zarqawi, calls for the release of kidnapped aid worker Margaret Hassan in Iraq unless she is proven to be what it calls 'a collaborator'.
