By Margarita Antidze

TBILISI (Reuters) - Georgia's new prime minister aims to change the constitution to boost his powers before a presidential vote next year, he said on Thursday, but will not try to impeach rival President Mikheil Saakashvili.

Bidzina Ivanishvili won power in a parliamentary election last month that ended Saakashvili's nine-year political dominance but left Georgia with an uneasy "cohabitation" government, with the president still in place, at least until October.

"We are not going to initiate the (president's) impeachment," Ivanishvili told a news conference, adding: "We will probably initiate constitutional amendments which will be clear to everyone."

Under Ivanishvili, authorities have arrested more than 10 former senior officials, including an ex-interior minister and the army chief-of-staff, raising opposition fears of a purge and drawing criticism from Saakashvili's allies in the West.

Parliament approved the constitutional changes - giving more powers to the legislature and the prime minister - before the election which brought the billionaire businessman to office - but they were due to take effect only after the October 2013 presidential election.

"We want these amendments to come into force earlier," Ivanishvili said. "Not after the election in October 2013, but earlier."

Ivanishvili signaled that relations with the West were a priority by making Brussels his first foreign trip last week. But he said a planned visit to the United States would be postponed until next year.

NATO and European Union leaders upbraided the prime minister over the arrest of political opponents as he visited Brussels and Ivanishvili said afterwards that his four-day absence had harmed the government's work.

The 56-year-old made his fortune mainly in Russia and faces a difficult balancing act between the West and Moscow, which welcomed his election victory. He ruled out a visit to Russia, but said he might meet Russian officials in some neutral territory.

"My visit to Russia is impossible as there are Russian embassies in (disputed territories) Abkhazia and South Ossetia ... but there might be meetings in Europe or somewhere else, although it's too early to talk about it," Ivanishvili said.

Saakashvili secured a promise of eventual NATO membership for his country of 4.5 million, but the alliance rejected calls led by former U.S. President George W. Bush for quick accession in 2008.

Months later, Georgia fought a five-day war with Russia over the breakaway regions of South Ossetia and Abkhazia, which Russia says are now independent states.

(Reporting by Margarita Antidze; Editing by Robin Pomeroy)