* Kiev says 41,000 Russian troops grouped nearby border with Ukraine
* Surveys show growing support for joining NATO among Ukrainians (Adds higher call-up age ahead, detail of Tuesday fighting, surveys on NATO)
By Natalia Zinets
KIEV, July 22 (Reuters) - Ukraine's parliament approved a presidential decree on Tuesday to call up more military reserves and men under 50 to fight rebels in eastern Ukraine and defend the border against a concentration of troops in Russia.
Some 45 days after the latest call-up of additional reserves, which has now expired, Kiev repeated the decree to "declare and conduct partial mobilisation" to ensure the ranks of what Ukraine calls its "anti-terrorist operation" are filled.
Ukrainian troops have forced pro-Russian rebels back to their two main strongholds, the cities of Donetsk and Luhansk, slowly taking villages and city suburbs around them.
On Tuesday, heavy fighting went on around the towns of Severodonetsk and Lysychansk near Luhansk and a security official in Kiev said Ukraine's army was aiming to close in on the rebels in Luhansk in the coming days.
The army is under orders not to use air strikes and artillery in the cities, complicating operations to restore control despite Kiev's accusations that the rebels were responsible for the shooting down of a Malaysian airliner. The separatists deny the accusations.
"Russia continues its policy of escalating its armed confrontation," Ukraine's top security official, Andriy Parubiy, told parliament before 232 deputies in the 450-seat parliament voted in favour of the decree.
Reiterating accusations levelled by Ukrainian officials against Moscow, he said: "Over the last week, close to the Ukrainian border, there has been a regrouping and build-up of forces of the Russian Federation."
Parubiy put the numbers close to the border at 41,000 and said they were equipped with 150 tanks, 400 armoured vehicles and 500 other weapon systems.
He said some of the new Ukrainian recruits would join or support combat units and some of the others would support units to help defend the border.
GROWING SUPPORT FOR NATO
Separately, the parliament also sought to raise to 60 the maximum age of Ukrainians who may be called up from military reserves in the future from the current ceiling of 50. The change needs to be approved by the president and it was not immediately clear if it would apply to the Tuesday call-up.
Russia withdrew most of the 40,000 troops it had close to the border earlier this year, reducing them to fewer than 1,000 by mid-June. But since then, it has been building up its forces again, a NATO military officer said this month.
Parubiy accused Russia of continuing to supply the rebels, who say they are fighting to win independence from Kiev for the Donbass coal mining region.
"Such actions are classified as aggression against our state," he said.
Moscow denies supplying the rebels.
The violent stand-off between Kiev and pro-Russian rebels in the east, which started in April, increased support among Ukrainians for joining the European Union and NATO, an opinion poll conducted by the Raiting Group pollster showed on Tuesday.
The survey, conducted at the turn of the month, showed 44 percent in favour of joining the Western military alliance, versus 40 percent in April. The number of those opposed stood at 35 percent and was down from 46 percent in April, it said.
A different poll by the Razumkov think-tank in Kiev in early June put support for joining NATO at 40.8 percent compared to 40.1 percent against. The pollster said the roughly equal figures mark the highest level of backing for becoming a NATO member recorded in years.
Russia vehemently opposes any NATO enlargement in the former Soviet bloc, which Moscow sees as a sphere of special interest. Kiev says joining NATO is not on the agenda now, and there is no suggestion from the alliance that Ukrainian membership is a near term possibility. (Writing by Elizabeth Piper and Gabriela Baczynska; editing by Janet McBride)