COLOMBO, April 25 (Reuters) - A Sri Lankan state-backed former Tamil Tiger rebel group, accused of abductions and killings, has freed 28 child soldiers, UNICEF said on Friday, welcoming the second major release in two weeks.

The TMVP, made up of fighters who defected from the mainstream Tamil Tigers in 2004 and helped the government evict their former comrades from the island's east, released 11 children in early April.

"We are beginning to see significant progress from the TMVP," Philippe Duamelle, head of the U.N. children's agency UNICEF in Sri Lanka, was quoted as saying in a press release.

"If the TMVP continues to release at this rate, and refrains from recruiting, they will eventually confirm with their public commitments to release all children," he said.

UNICEF accuses the Liberation Tigers of Tamil Eelam (LTTE) rebels and the former rebels, the Karuna group or the TMVP, of abducting children or forcibly recruiting them.

The UN agency estimates more than 5,600 underage fighters have been recruited or re-recruited in Sri Lanka, the vast majority by the Tamil Tigers, since a 2002 ceasefire in the 25-year civil war broke down in 2006.

The average age of child recruits in 2007 was 16 years according to the United Nations, though some children much younger were recruited at earlier stages of the conflict. About 40 percent were girls.

Tamileela Makkal Viduthalai Pulikal, or TMVP, the Karuna group's political arm, secured a landslide victory at its first election contest in March in the eastern district of Batticaloa.

The Sri Lankan government, itself increasingly isolated over its human rights record, is under pressure to disarm the TMVP and push for the release of the child soldiers.

President Mahinda Rajapaksa's administration has long refused to disarm the TMVP, arguing it could not find anyone carrying guns to disarm -- despite the fact that residents and aid workers could until a few months ago. (Reporting by Ranga Sirilal; Editing by Valerie Lee)

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