(Recasts with Georgian president's statement, Russian reply, changes dateline, previous Tbilisi)

By Paul Taylor

UNITED NATIONS, Sept 26 (Reuters) - Georgia and Russia clashed at the United Nations on Wednesday over an incident in which Georgian security forces killed two Russian officers training insurgents in the breakaway region of Abkhazia.

Georgian President Mikhail Saakashvili told the U.N. General Assembly that one of those killed in the Sept. 20 "law enforcement operation" was a Russian army lieutenant-colonel.

Pointing an accusing finger at Moscow, he said: "One has to wonder -- what was a vice-colonel of the Russian military doing in the Georgian forests, organizing and leading a group of armed insurgents on a mission of terror?"

Russian U.N. envoy Vitaly Churkin told reporters Georgian troops had made an unprovoked attack on an "anti-terrorist training" exercise in a camp in the Kodori gorge and finished off two Russian instructors with knives and shots to the head.

"They have done everything to aggravate tensions," Churkin said. The Kodori gorge, a gateway to the Black Sea province of Abkhazia, has enjoyed de facto independence since 1993.

Relations between Moscow and the former Soviet republic have been tense ever since Saakashvili came to power in a U.S.-backed "Rose Revolution" in early 2004.

Russia cut transport and trade ties with Georgia last year after Tbilisi arrested four Russian military officers accused of spying and paraded them on television.

In August, Georgia accused Moscow of dropping a missile on its territory. The Russian airforce has denied it violated Georgian air space.

"This reckless and dangerous pattern of behaviour must not continue," Saakashvili said in his U.N. speech.

MORTAR FIRE

Whatever the explanation for the latest Abkhazia incident, he said, Georgia regretted any loss of life and would do everything possible to "avoid violence and further hostility and confrontation.'

Abkhazia is not the only region where Russian-backed separatists are holding out against the central government.

Heavy mortar fire erupted in the breakaway province of South Ossetia on Wednesday, but the Georgians and separatists disagreed over whose troops were responsible.

Mamuka Kurashvili, the commander of the Georgian peacekeeping battalion in Ossetia told Reuters by telephone that separatists had opened fire from the regional capital, Tskhinvali, and hit government-controlled villages nearby.

The separatists said Georgian forces opened fire on Tskhinvali and they had to respond.

A separatist Web site, coninf.org, said one woman was wounded, but Russian media quoted Ossetian officials as saying that between two and four people were wounded.

The Web site said Ossetian President Eduard Kokoity had sent heavy armour to the area.

Ossetia broke away from Georgia soon after the collapse of the Soviet Union in 1991. A joint Georgian-Russian peacekeeping force polices the shaky truce in the region, where bouts of violence are not unusual.

In a sign of how sensitive any violent flare-up is in the region, Abkhazia's leader Sergei Bagapsh held an urgent telephone conversation with Kokoity and told his troops to move closer to the border with Georgia proper. (Additional reporting by Margarita Antidze in Tbilisi and Claudia Parsons at the United Nations)

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