By Louis Charbonneau

UNITED NATIONS, April 15 (Reuters) - The United States and Russia exchanged sharp words over Georgia's two breakaway regions on Tuesday after the U.N. Security Council unanimously extended the mandate of U.N. military observers in Abkhazia.

U.S. Ambassador to the United Nations Zalmay Khalilzad told reporters Washington was "deeply concerned by reports from Moscow that Russia is planning on establishing semi-official representative missions in Abkhazia and South Ossetia without the approval of the Georgian government."

"We urge Russia not to follow this path, which would undercut Russia's stated support for the principles of Georgian sovereignty and territorial integrity," he said.

The Abkhazia and South Ossetia regions broke away after the 1991 collapse of the Soviet Union, when Georgia became independent. They have since relied on Moscow's support.

The resolution, which Russia, the United States and four other countries co-sponsored, extended the mandate of the U.N. Observer Mission in Georgia (UNOMIG) until Oct. 15.

It also said the Security Council "reaffirms the commitment of all (U.N.) member states to the sovereignty, independence and territorial integrity of Georgia."

Russia says it respects Georgia's integrity but Abkhazia and South Ossetia have a legitimate right to seek independence after the West recognized Serbia's breakaway region of Kosovo over Belgrade's objections.

Russia's U.N. envoy Vitaly Churkin accused Khalilzad of "completely missing the real problem of the Georgia-Abkhazia conflict" including Georgian military activity in Abkhazia.

"There is the new phenomenon of overflight jets (in) the security zone," he said. "Recently a Georgian drone was shot down in the airspace of the security zone, there is a buildup of the Georgian military."

RUSSIA SUPPORTS ABKHAZIA

Khalilzad said Kosovo and Georgia were entirely different cases. He also reiterated U.S. criticism of Russia's decision last month to drop trade restrictions against Abkhazia.

Churkin said that decision was in line with the resolution passed on Tuesday, which mentions the need for measures that would improve the economic situation in Abkhazia.

"This is of course the target of the steps taken by the Russian Federation in lifting the outdated sanctions," he said, adding that there might eventually be commercial Russian flights to the Abkhazian capital, Sukhumi.

Georgia has angered Russia by applying to join the NATO military alliance. It said it wanted good relations with Russia and a negotiated settlement with the two breakaway regions.

So far the separatists have rejected Tbilisi's offer of a power-sharing deal, saying they want full independence. (Editing by Alan Elsner)

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