NAIROBI, Kenya - A push by Kenyan ground forces toward a strategic Somali town has been slowed by heavy rain, a military spokesman said Tuesday, as Kenya's defence and foreign ministers arrived in Mogadishu for talks.

Kenyan troops and pro-Somali government forces are heading toward the town of Afmadow, a crossroads town just north of the port city of Kismayo, where the militant group al-Shabab is entrenched.

Maj. Emmanuel Chirchir said Kenyan forces were at the Somali town of Qoqani, about 80 kilometres from Afmadow. Residents of Afmadow on Monday reported that al-Shabab fighters were leaving as the troops approached, though Chirchir said progress has been slowed by rain.

"Our troops are heading to Afmadow now, and we expect to capture it either today or tomorrow," Abdinasir Serar, a commander with the pro-government Ras Kamboni militia told The Associated Press by phone from Qoqani.

In Mogadishu, Kenya's ministers of defence and foreign affairs were to meet with the leaders of Somalia's Transitional Federal Government, said government spokesman Abdirahman Omar Osman.

Kenyan forces moved en masse into Somalia over the weekend, following a declaration on Saturday by Kenyan leaders that the country had the right to defend itself. Osman said previously that Kenyan troops were not needed in Somalia's south.

Somali gunmen over the last six weeks have kidnapped four Europeans and killed a fifth inside Kenya. Al-Shabab on Monday denied involvement in the abductions, and warned Kenya of terrorist attacks on its soil unless its forces retreat.

Though Kenya said the kidnapping attacks were the catalyst for the invasion, there are indications the push into Somalia has been in the works for some time. Military analysts say it is highly unlikely Kenya could organize such a complex military operation so quickly in response to the kidnappings.

The Kenyan invasion comes at a time when al-Shabab has been weakened by famine in its strongholds, has been pushed from the capital of Mogadishu by African Union troops and finds itself increasingly challenged by clan militias.

Kenya moved two battalions of about 800 troops each across the border in two locations, a Nairobi-based official said. Tanks, helicopters and artillery have also been deployed. The invasion is the most significant foreign deployment of the Kenyan military since independence from Britain in 1963.

Al-Shabab threatened on Monday to bring down Nairobi skyscrapers and referenced the July 2010 bomb attacks they masterminded in Kampala, Uganda, that killed 76 people. Al-Shabab said the attacks were retaliation for Uganda's troops contributions to the African Union peacekeeping force in Mogadishu.

"Remember what happened in Uganda's capital," Sheik Ali Mohamud Rage, an al-Shabab spokesman, said Monday.

Kenya's final objective remains unclear. It has spent the last two years pushing for a buffer zone between it and troubled Somalia. Kenyan forces trained and equipped the so-called Jubaland militia of more than 2,000 Somalis and have frequently said they want to take Kismayo, a port city whose customs revenues are the insurgency's biggest cash cow.

Al-Shabab's key line of defence for Kismayo is in front of the Juba river. There are only three bridges across it strong enough to take the movement of vehicles.