* No gunfire overnight in pro-Ouattara neighbourhood
* Ivory Coast in turmoil since disputed November election
* Relations between Gbagbo and U.N. mission deteriorating
By Stephanie Nebehay and Ange Aboa
GENEVA/ABIDJAN, Jan 13 (Reuters) - The United Nations&${esc.hash}39; human rights chief raised the alarm on Thursday over a suspected third mass grave in Ivory Coast, as tensions remained high in the main city Abidjan after deadly clashes between rival camps.
The world&${esc.hash}39;s top cocoa grower has been locked in a violent power struggle since a Nov. 28 election which both the incumbent Laurent Gbagbo and rival Alassane Ouattara claim to have won.
U.N. human rights chief Navi Pillay said Ivorian forces were blocking access to a site near the central town of Daloa where a mass grave had been reported, as they have already been accused of doing at two other sites in and around Abidjan.
"I am very concerned now that a third mass grave has been discovered. Not only my representative there but the U.N. representative has not been allowed access to the mass graves," she told Reuters in her office in Geneva.
The election was meant to heal a nation divided into a government-held south and a rebel-run north by a 2002-3 war, but has instead only deepened divisions.
More than 200 people have died in violence since the vote, and fears of more conflict have prompted more than 20,000 people to flee into neighbouring Liberia, according to U.N. figures.
The U.N. suspects many of the dead were killed by pro-Gbagbo security forces or allied militias in night-time raids on neighbourhoods, with hundreds more abducted.
Gbagbo&${esc.hash}39;s camp has repeatedly dismissed reports of mass graves and "death squads" as fabrications by Ouattara allies.
"LIVING IN FEAR"
Relative calm was restored overnight in the Abidjan suburb of Abobo, a day after clashes there between security forces loyal to Gbagbo and supporters of Ouattara killed six policemen and some civilians. Five died in another battle on Tuesday.
Gbagbo&${esc.hash}39;s interior ministry said some of the police who died in Wednesday&${esc.hash}39;s clashes had been killed when their vehicle was attacked with rocket-propelled grenades. Ouattara&${esc.hash}39;s camp said Gbagbo&${esc.hash}39;s forces had killed at least seven civilians.
In a fresh incident underlining growing tensions between Gbagbo supporters and the 10,000-strong U.N. peacekeeping mission UNOCI, pro-Gbagbo students attacked and burned a U.N. pick-up truck in Abidjan on Thursday.
"It&${esc.hash}39;s an opportunity for us to show that with our bare hands we can also take action," said student Max Amani. "Each day we hear shooting in Abobo we will burn 10 UNOCI cars," he said as others jumped up and down on the burnt-out vehicle.
Reuters witnesses saw a dozen armoured vehicles ferrying troops with automatic weapons on patrol in the northern and largely pro-Ouattara neighbourhoods of Abobo and Ayaman. Soldiers manned makeshift checkpoints to control traffic.
"The people still live in fear in Abobo," said resident Ladji Bakayoko.
Ouattara was proclaimed the winner by the electoral commission and is widely regarded by foreign governments as having legitimately won the U.N.-certified poll.
But Gbagbo has refused to concede, with backing from the top court, and controls the security forces and allied militias, which U.N. officials suspect of rights abuses. The stand-off has sent cocoa prices to four-month highs in recent weeks.
Early on Thursday Mangou said he blocked U.N. peacekeepers from entering. "According to our last report from the field, the head of the (U.N.) operation was turned around and went back," Mangou told journalists after meeting Gbagbo at his residence.
"They are supposed to be an impartial force ... to bring us peace, but they have become something else." However a U.N. mission statement said its chief Y.J. Choi had gained access to Abobo on Wednesday night.
(Additional reporting by Tim Cocks and Loucoumane Coulibaly in Abidjan; writing by David Lewis; editing by Andrew Roche)
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