* Clashes caused 491 confirmed deaths, 108 unconfirmed
* Violence spiked after JEM rebels froze talks
* Aid work hit as humanitarian flights blocked (Adds detail, background, aid comment)
By Andrew Heavens
KHARTOUM, June 7 (Reuters) - Nearly 600 people died in rebel and tribal fighting in Sudan's Darfur region in May, the bloodiest month that the territory has seen in more than two years, U.N. officials said on Monday.
Violence in the seven-year-old conflict has spiked since one of the main rebel groups, the Justice and Equality Movement (JEM), announced in early May it was freezing its participation in peace talks.
The death toll underlined the challenge facing mediators seeking to resolve a conflict that has continued in the face of pressure from Washington, war crimes prosecutions by the International Criminal Court and campaigning by activists.
"There were 491 confirmed deaths and 108 unconfirmed deaths," said one official from Darfur's joint U.N./African Union UNAMID peacekeeping force, adding it was the largest death count recorded since the force was set up in January 2008.
The data was released as Sudan's government was due to resume troubled peace talks with an umbrella group of small Darfur insurgent factions in Doha, capital of Qatar.
"The figure is very high. This was caused by tribal fighting and fighting with JEM," said another UNAMID official. Both officials spoke on condition they not be identified.
JEM announced last month it was freezing its participation in the Doha talks, protesting against the involvement of other insurgents in the negotiations and accusing Khartoum of breaking an earlier ceasefire.
Sudanese army forces said they drove JEM out of its stronghold close to the border with Chad later in May, forcing it to move units into central and southern Darfur, as well as the neighbouring oil-producing state of Southern Kordofan.
Senior JEM official al-Tahir al-Feki said on Monday it would continue to boycott the negotiations and was clashing continuously with Sudanese army forces. No one was immediately available for comment from Sudan's army.
The May figures were about five times the average monthly death counts recorded in 2009 said Sudan expert Alex de Waal who highlighted the increase on his blog Making Sense of Sudan (blogs.ssrc.org/sudan/).
Aid workers who set up the world's largest humanitarian operation in Darfur said their work was being hampered by the insecurity and an increase in bandit attacks and kidnappings.
"For about one week we haven't had access to deep field locations in Darfur because it is not safe out there," said Marian Yun from the U.N.'s World Food Programme.
Other aid workers said Sudanese and U.N. security officers had banned flights in south Darfur, excluding journeys to and from regional capital Nyala, leaving some staff stranded in remote settlements.
JEM was one of two mostly non-Arab rebel factions that took up arms against Sudan's government in 2003, accusing it of neglecting the development of the region.
Khartoum mobilized mostly Arab militias to crush the uprising. Washington and some activists accused Sudan of launching a genocidal counter-insurgency campaign that targeted many civilian communities. Sudan's government dismisses the accusation.
The International Criminal Court last year issued an arrest warrant for Sudanese President Omar Hassan al-Bashir to face seven charges of masterminding war crimes and crimes against humanity in the mostly desert region. (Editing by Jon Boyle)