The Sri Lankan government began escalating its military assault on the LTTE after a ceasefire broke down in January and the Sri Lankan army now claims it is close to capturing Kilinochchi, the Tiger's de-facto capital.
The UN estimates as many as 230,000 people have been displaced around Kilinochchi and the stronghold of Mullaittivu, in Sri Lanka's Vanni region.
"The fighting has become increasingly severe in the past three months - it affected our own operation in the area which is why we withdrew," Gordon Weiss, who is helping organise the UN relief efforts in Sri Lanka, told Al Jazeera.
He said the renewed aid convoys into the Vanni required the co-operation both the government and the LTTE.
"The government, in fact, is intimately involved in this convoy - they support the effort to move supplies into the Vanni ... The government is making up half of the convoy that is going into the Vanni this week," Weiss said.
"It's not going to be sufficient in and of itself but its the first of a number of convoys. This is going to be a supply line."
Sri Lanka's government ordered aid workers to leave the country's northern war zone at the beginning of September, amid intense fighting in the Tamil Tiger-held north.
The government said it was trying to avoid a repeat of the August 2006 killing of 17 local aid workers employed by the French aid agency Action Against Hunger.
Land and sea battles in the north have killed at least 53 people in clashes between the Tigers and the Sri Lankan army, the military said on Tuesday.
In an ocean battle, the Sri Lankan navy said it blew up two Tiger boats and killed at least eight "Sea Tiger" fighters.
Land clashes in the Indian Ocean island nation's north on Tuesday killed 39 fighters and six soldiers, while wounding 52 LTTE fighters and 19 soldiers, the military said.
It was not possible to verify the military's reports because most journalists and other independent observers are barred from the war zone.
Both sides often exaggerate their enemy's losses and under report their own.