* Constitution drafting may lead to new parliament
* Libya's congress deadlocked between two blocs
* Army commander's son kidnapped in Benghazi (Adds details from clashes and on transition)
By Ghaith Shennib
TRIPOLI, Jan 30 (Reuters) - Libya will elect an assembly on Feb. 20 to draft a constitution intended to advance transition to democracy and break political stalemate more than two years after a NATO-backed uprising toppled Muammar Gaddafi.
The North African country is caught up in chaos with its Congress deadlocked between Islamists and a leading nationalist party, and its nascent army struggling to assert itself against unruly former rebels, tribal groups and Islamist militants.
Just hours before the congress decision, gunmen kidnapped the son of Libya's special forces commander in Benghazi, later calling the colonel to demand he withdraw troops in return for his son's release, state news agency LANA said.
At least one soldier was killed, medical and security sources said, after troops clashed with gunmen in the eastern city. Militants from the hardline Islamist Ansar al-Sharia group have been fighting troops there.
Setting the date for the vote was a small step in a transition that has frustrated Libyans since the fall of Gaddafi with the fragile government often at the mercy of rival bands of former rebels who refuse to accept Tripoli's authority.
"We want all Libyan people and groups to reconcile and support these elections," Nouri Abusahmain, president of the General National Congress, said after announcing the date of the vote On Thursday.
Once the 60-member constitutional assembly is elected, it will have 120 days to draft a new charter, which would then be submitted to a popular referendum. If the document is approved, an election for a proper parliament would be held in late 2014.
But that drafting process is likely to be complicated by the demands of tribal, regional and ethnic interest groups already vying for influence over Tripoli's government.
The constitutional drafting has already been delayed by infighting between the National Forces Alliance party, and the Islamist Justice and Construction party, the political arm of the Muslim Brotherhood in Libya.
The transitional term of the General National Congress was to have expired on Feb. 7, but its members extended this to provide continuity, promising to stay on only if progress on the constitution is being made.
Political divisions in Libya are accompanied by rivalries among brigades of former fighters and militants and tribal groups who fought Gaddafi, and now want a greater share in the post-revolution spoils and power. (Additional reporting by Ayman al-Wafalli in Benghazi; Writing by Patrick Markey; Editing by Alistair Lyon and Mohammad Zargham)