Abdou Dardar, from the Ansar Dine group, said fighters from the Islamist force were in the town of Konna, northeast of the regional capital of Mopti, and witnesses later in the day said Malian soldiers were retreating.
The apparent rout came amid other clashes between government forces and Islamist rebels in the region, marking a dramatic new phase of the crisis that until now had seen the Islamists stay largely in Mali's vast arid north.
"We almost entirely control the town (of Konna.)Afterwards, we are going to continue" pushing south, Dardar said by phone.
Dardar said he was speaking in the name of all the Islamists. Witnesses told that Malian troops were retreating toward Sevare, near Mopti.
Diplomatic sources at UN headquarters in New York said the 15-member Security Council would meet after they heard Islamist guerrillas were within 20 kilometres of Mopti, the gateway between rebel-held and government-held areas.
During a brief visit to Bamako yesterday, UN special representative for the Sahel, Romano Prodi, said the Islamist push was of serious concern and could lead to "extraordinary"decisions from the international community.
He didn't elaborate, and stressed his presence in the Mali capital was unrelated to the developments.Tensions were running high in Bamako, following a day of street protests Wednesday.
The north has been controlled for nine months by Al-Qaeda in the Islamic Maghreb (AQIM), the Movement for Oneness and Jihad in West Africa (MUJAO), and Ansar Dine (Defenders of the Faith), who all promote the application of Islamic law.
Alexis Kalambry, a Malian journalist and political analyst, told that Mali's army has wanted to attack the north since last month but has refrained from taking action"due to Mali's allies." The presence of hardline armed Islamists in Mali's remote desert terrain has aroused fears among regional states and the international community that the north -- an area about the size of France -- could become a launch-pad for Al-Qaeda activity.