(Adds Saakashvili; confirmation of U.N. envoy's resignation)

By Margarita Antidze and Matt Robinson

TBILISI, Dec 5 (Reuters) - Georgian Prime Minister Grigol Mgaloblishvili dismissed the defence and foreign affairs ministers on Friday in a major shakeup following August's military defeat by Russia.

The changes come amid opposition criticism of President Mikheil Saakashvili's government and its loss in the five-day war that followed Tbilisi's military assault on its breakaway South Ossetia region.

"Changes need to be made in the sphere of defence," Mgaloblishvili told a news conference. "It is important to strengthen foreign policy and more experience is needed in this direction."

The opposition accuses Saakashvili of walking into a war former Soviet Georgia could not possibly win, leaving the military devastated and the economy deprived of foreign private investment just as the global financial crisis takes hold.

Saakashvili insists Russia was the aggressor, sending tanks and troops into its southern neighbour in support of pro-Moscow separatists. But diplomats say Georgia's standing with its Western allies is clearly shaken.

The reshuffle coincided with the resignation of Georgia's U.N. ambassador. Ambassador Irakly Alasania regularly polls among Georgia's most popular public figures and has been touted as a potential challenger to the president.

In a television interview, he did not say why he had quit, but it leaves him free to join the opposition.

Mgaloblishvili, who was appointed by Saakashvili in late October, said 30-year-old Defence Minister David Kezerashvili would be replaced by his deputy Batu Kutelia.

Saakashvili had already dismissed the military chief of staff, who admitted flaws in the conduct of the war and the chaotic retreat by security forces from Georgia's two breakaway regions, South Ossetia and Abkhazia.

Mgaloblishvili, 35, named Culture Minister Grigol Vashadze as the new foreign minister, replacing 31-year-old Ekaterine Tkeshelashvili.

During a televised meeting with the prime minister, Saakashvili said the government had performed well, but "new blood" was needed.

"We face the dual burden of the global financial crisis and the consequences of war with Russia," he said. "This calls for courageous and extraordinary decisions."

Several former close allies of Saakashvili have swapped sides. Nino Burjanadze, a co-author with Saakashvili of the 2003 "Rose Revolution" that brought down longtime leader Eduard Shevardnadze, last month formed her own opposition party.

Saakashvili has promised democratic reforms to answer critics who say he has fallen short on early promises of political, judicial and media freedom after the 2003 revolution.

Analysts have called on him to broaden the reforms and carry them through if his government is to weather the economic crisis and make the most of billions of dollars in international aid, or face rising social discontent through the winter.

Tens of thousands of Georgian refugees from the August war have yet to return to their homes. Russia, which said it intervened to protect civilians from the Georgian assault, has recognised South Ossetia and Abkhazia as independent states. (Additional reporting by Niko Mchedlishvili; editing by Michael Roddy)

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