The towns of Puliyankulam and nearby Puthur Junction fell to Tiger rebels on Saturday with heavy casualties, officials said.

Overnight, the rebels of the Liberation Tigers of Tamil Eelam (LTTE) also took Kanakarayankulam and a military base at Nainamadu further east.

Heavy artillery duels are being fought as the army reinforces itself with thousands more soldiers.

Territory recaptured

The military has lost nine of its key bases in the northern Wanni region in five days. It had taken government troops more than two years to capture the area.

The military had intended to take control of the A9, which cuts through the Tiger held area to Jaffna, but abandoned its offensive in December with the loss of 1,500 troops.

Puliyankulam is 24 km (15 miles) north of the government-held city of Vavuniya.

A statement faxed from theTigers' London office said that their latest advances meant that Vavuniya was now exposed.

Vavuniya lies some 220 km (135 miles) northeast of the capital Colombo and has a substantial minority Tamil population.

The BBC's Susannah Price in Colombo said before the latest fighting, there had been concerns that the military was overstretched and didn't have enough manpower to hold on to the area it had taken.

The BBC's Susannah Price reports: "Fighting has been intensive"

The Tigers said some soldiers were deserting their posts and fleeing towards Vavuniya.

Reports from there say there are large numbers of soldiers in the town and one group recently returned from battle field have been demanding cash and food from local shopkeepers.

The rebels said they have lost 100 fighters, and the army says 89 soldiers have been killed and 645 wounded.

Military sources say the casualties are much higher.

Political impact

In Colombo, the rival political leaders are continuing their campaign for next months' presidential elections, avoiding any direct reference to the fighting.

Analysts say the army losses could hurt President Kumaratunga's chances of re-election.

The Tigers say their offensive, codenamed "Ceaseless Waves Three", was planned and directed by the Tiger leader Velupillai Prabhakaran.

The LTTE is focussing on army bases west of its stronghold of Mullaitivu, on Sri Lanka's north-eastern coast.