June 5 (Reuters) - The United Nations said on Friday it is deeply concerned by violent protests outside its offices in Libya this week and blamed social media for fueling disinformation about the world body's work in the country.

Hundreds of Libyan demonstrators blocked off the office of the U.N. refugee agency (UNHCR) in Tripoli on Thursday during a protest against migrants who travel to Libya in search of work or passage to Europe.

It was the largest of several recent demonstrations against migrants, whom some Libyans blame for social and economic problems that have become more visible during 15 years of conflict and political division in the North African country.

"We are deeply concerned by the violent protests that took place yesterday outside the UNHCR and UNSMILheadquarters in Tripoli," U.N. spokesperson Stephane Dujarric told a regular briefing at U.N. headquarters in New York.

"The U.N. strongly condemns any attack or threats against personnel or premises," he said, adding: "We are concerned by the spread of misinformation and disinformation regarding the work of the United Nations in Libya."

Dujarric said countering disinformation in Libya was "extremely challenging" and called for cooperation from social media platforms to address the problem.

UNSMIL - the United Nations Support Mission in Libya - said on Thursday there is no U.N. resettlement program in Libya and that UNHCR works to help people fleeing war with solutions outside the country, including evacuation to third countries and voluntary return when conditions allow.

Since a NATO-backed uprising in 2011, Libya has become a transit route for hundreds of thousands of migrants fleeing conflict and poverty, often in sub-Saharan Africa, with many risking dangerous journeys across the desert or the Mediterranean.

The oil-dependent Libyan ​economy is also a draw for migrants seeking work, and many do menial jobs in sectors including cleaning and construction that Libyans are reluctant to fill.

(Reporting by David Brunnstrom; Editing by Sanjeev Miglani)