* Ukraine's interim PM tries to reassure Russia on NATO
* Yatseniuk says Ukraine to disarm all armed groups
* Says a new constitution will devolve powers to regions (Adds details, quotes)
By Pavel Polityuk
KIEV, March 18 (Reuters) - Ukraine's new pro-Western leadership is not seeking membership of NATO, Prime Minister Arseny Yatseniuk said on Tuesday, in comments intended to reassure Russia and Ukraine's large number of Russian-speakers.
Yatseniuk, who came to office after the removal of Moscow-backed President Viktor Yanukovich, also said decentralisation of power was a key plank of government policy, adding Kiev's efforts to integrate with Europe would take into account the interests of Ukraine's mainly Russian-speaking industrial east.
"Strictly with a view to maintaining Ukraine's unity, the question of joining NATO is not on the agenda," Yatseniuk, who normally speaks in Ukrainian, said in a 10-minute televised appeal delivered in Russian. "The country will be defended by a strong, modern Ukrainian army."
Kiev pursued a policy of closer ties with the U.S.-led NATO alliance before Yanukovich took power in 2010. Yanukovich then formally scrapped the idea of Ukraine's eventual membership of NATO, declaring 'non-bloc' neutrality for his nation of 46 million sandwiched between Russia and the European Union.
Clashes between protesters and police in which more than 100 people were killed in Kiev, many by police snipers, culminated in Yanukovich fleeing and later being ousted by a parliamentary vote in a move Russia has denounced as illegitimate.
Yatseniuk also said Ukraine's authorities were determined to disarm all groups holding weapons, regardless of their aims.
"This operation concerns all those holding arms illegally, whatever their slogans," Yatseniuk said, standing next to a blue and yellow national flag. "There can be no justification for holding arms illegally."
Ministers will extend greater powers to the regions, a key demand in mainly Russian-speaking eastern Ukraine, and a municipal police force will be created, Yatseniuk said.
"All the new measures linked to decentralisation of power will be reflected in a new constitution," he said. "A constitution that we will write together."
Russia, whose forces control Ukraine's Crimea region, says Yanukovich remains the legitimate president and denounces Kiev's new authorities in Kiev as anti-Russian and beholden to extreme right-wing groups who it says forced Yanukovich to quit.
Moscow says it reserves the right to "defend" Russian-speakers in eastern and southern regions of Ukraine.
Yatseniuk said Ukraine would sign on Friday the political part of an association agreement with the 28-nation EU, but would put off agreement on economic issues. It was Yanukovich's decision not to sign the association accord with the EU and to pivot towards Moscow that ignited the street protests in Kiev last November that finally led to his ouster.
"We have postponed for the moment signature of the economic part of this document, taking into consideration the concerns and dangers of whether a free trade zone will have negative effects on industrial regions, primarily the east," he said.
Moving closer to the EU does not preclude Ukraine maintaining good relations with Russia, Yatseniuk added.
"Despite the catastrophic worsening of relations with Russia, occurring through no fault of our own and despite Russia's armed aggression against Ukraine, I will do everything possible to uphold peace and build relations of partnership and good neighbourliness." (Writing by Ron Popeski; Editing by Richard Balmforth and Gareth Jones)