Baidoa, Somalia - Somalia's prime minister accused the lawless nation's powerful Islamist movement on Wednesday of planning to attack the seat of the weak transitional government, raising already heightened tensions.

Prime Minister Ali Mohamed Gedi said Muslim militia had moved to within 40km of the government's base in Baidoa, north-west of Mogadishu, and intended to strike the town in violation of a truce.

The Islamists furiously denied the charge, but allowed that forces loyal to Supreme Islamic Council of Somalia (SICS), which seized the capital from a United States-backed warlord alliance last month after fierce battles, were near Baidoa.

Gedi said "several" Islamic fighters with around 30 battlewagons -- machine gun-mounted pick-ups also known as "technicals" -- had positioned themselves in the town of Buurhakana, south-east of Baidoa, and were plotting an attack.

"It's very clear they are planning to attack the transitional federal government," he said. "The national security forces are on high alert in order to protect the government, Baidoa town and people of Somalia."

"They arrived in Buurhakana last night and their movement continues," Gedi told Agence France-Presse (AFP) in an interview. "They are going back and forth, most probably bringing in arms."

But a senior SICS cleric in Mogadishu rejected the claim, saying 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.

"We have no intentions to attack Baidoa. The allegations of the government are unfounded," the cleric told AFP. "We are consolidating our power in areas we control," he said.

"Those who defected from Baidoa requested transportation to come to Mogadishu and we are facilitating," he told AFP on condition of anonymity.

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.

The rise of the Islamists has also alarmed the west, which fears "creeping Talibanisation" in Somalia and that matters could disintegrate and further engulf the country in turmoil, potentially starting a regional conflict.

It was not immediately clear if the new development would affect Arab League-sponsored talks aimed at calming the situation, which the government agreed to on Monday after boycotting a planned weekend session in Sudan.