By Margarita Antidze

TBILISI (Reuters) - Georgia's ruling parliamentary coalition tried to strip some of President Mikheil Saakashvili's powers on Thursday in the latest skirmish of a political struggle that could threaten supplies of Caspian oil and gas to Europe.

Billionaire Bidzina Ivanishavili has been trying to assert his supremacy over Saakashvili since he became prime minister last October, creating an awkward power-sharing arrangement.

The Georgian parliament began debating a motion to make constitutional changes watering down the president's powers, a move that is unlikely to secure the required two-thirds majority but is likely to increase tension between the two rivals.

Further instability in the country of 4.5 million, which fought a five-day war with Russia in 2008, would worry the West because of the threat to pipelines which carry Caspian natural gas and oil to Europe through Georgia.

The former Soviet republic also has an important strategic location on the Black Sea between Russia and Iran, Turkey and central Asia.

The prime minister enjoys more day-to-day authority in Georgia but the president has the authority to dismiss the government and parliament and to send laws back to the assembly.

Saakashvili, 45, whose term expires in October and who is barred by the constitution from seeking re-election, says he has no plans to dismiss Ivanishvili's government, which took office after an election last October.

"I was not going to dismiss the government, and I still have no intention of doing so," Saakashvili said on Wednesday after meeting his allies in parliament.

"The stability and development of our country is a top priority for me as president," he said.

PRESIDENT VS PRIME MINISTER

But Ivanishvili's allies want to curb the president's powers to be sure he cannot dismiss the government, even though some powers are already due to be transferred to the prime minister under constitutional changes agreed by the last parliament and due to take effect after the next presidential election.

"We don't want Saakashvili to have this stone in his pocket (to throw at the government)," Tina Khidasheli, a lawmaker from the Georgian Dream coalition, said.

The 57-year-old prime minister's allies in parliament are several votes short of being able to muster the two-thirds majority needed to change the constitution.

Failure to win support for the motion would be likely to end parliamentary efforts to curtail presidential powers but could aggravate tension between the two men.

Saakashvili's United National Movement party says it would be ready to support the constitutional amendments, but only if parliament first holds a non-binding test vote.

Closer cooperation with the European Union and NATO have long been priorities for Saakashvili, who in the past accused Ivanishvili of being a Russian stooge.

The prime minister has said relations with the United States and Europe remain the main priority, but that he still wants to improve ties with Russia. Talks are under way with Moscow on resuming imports of mineral water and wine from Georgia, which were halted in 2006.

(Reporting by Margarita Antidze; Editing by Timothy Heritage and Paul Casciato)