Prime Minister Ali Mohamed Gedi said Muslim militia had moved to within 40 kilometres of the government's base in Baidoa, northwest of Mogadishu, and intended to strike the town in violation of a truce.
The Islamists furiously denied the charge, but acknowledged that forces loyal to Supreme Islamic Council of Somalia were near Baidoa.
A senior SICS cleric in Mogadishu said Islamic gunmen were in Buurhakana only to collect 135 militiamen previously loyal to the government who defected and asked to be brought to the capital.
But Mr Gedi said "several" Islamic fighters with around 30 battlewagons had positioned themselves in the town of Buurhakana, southeast in Baidoa, and were plotting an attack.
US concerned
Washington expressed "grave concern" over the reports.
"The United States is gravely concerned by reports that militia aligned with the Islamic courts are advancing toward Baidoa, the interim locations of the Somalia Transitional Federal Institutions," said State Department spokesman Sean McCormack.
"This continued expansion contradicts previous statements by the Islamic courts regarding their intentions toward the Transitional Federal Institutions."
It was not immediately clear if the new development would affect Arab League-sponsored talks aimed at calming the situation that the government agreed to on Monday after boycotting a planned weekend session in Sudan.
Tension between the government and Islamists has run high since the Muslim militia took control of Mogadishu in June and have since expanded their rule, challenging the authority of the largely powerless government.