* Ukraine, Russia step up accusations over fighting
* Poroshenko says new Russian missile system in use (Releads with Poroshenko's remarks)
KIEV, July 14 (Reuters) - Ukrainian President Petro Poroshenko accused Russian military staff officers on Monday of fighting alongside separatists in the east of the country and said a newly-developed Russian missile system was being used against government forces.
Poroshenko was speaking at an emergency meeting of his security chiefs after a weekend of Ukrainian air strikes on rebel positions near the border with Russia and charges by Moscow that Kiev killed a Russian man with a cross-border shell.
The war of words between Kiev and Moscow and the intense fighting, in which Ukrainian forces say they inflicted heavy losses on the rebels, marks a sharp escalation in the three-month conflict in which several hundred Ukrainian servicemen, civilians and rebels have been killed.
"Information has ... been confirmed that Russian staff officers are taking part in military operations against Ukrainian forces," Poroshenko said, adding to his charges on Sunday of movements of heavy military equipment into the country from Russia.
He said Ukrainian forces were now coming under attack from a new Russian missile system and that Ukrainian forces would have change tactics on the border, though he gave no details.
Earlier on Monday, a military spokesman in Kiev said Russia was building up forces on its border with Ukraine, and separatists, backed by Russian "mercenaries", were firing on Ukrainian border guards in an attempt to bring armoured vehicles into the country.
Accusing Russia of embarking on a course of escalation in Ukraine's eastern regions, National and Security Council spokesman Andriy Lysenko told journalists:
"In the past 24 hours, deployment of (Russian) units and military equipment across the border from the Sumy and Luhansk border points was noticed. The Russian Federation continues to build up troops on the border."
In the early hours of Monday, separatists had fired on border guards and the armed forces near the border settlement of Dyakove, one of several attacks on border guards as "terrorists and Russian mercenaries" tried to bring in armoured vehicles and equipment, he said.
Lysenko accused rebel fighters of being behind the cross-border shelling of a Russian residential area in which a Russian man was killed on Sunday and which Moscow says was the work of Ukrainian forces.
"The (rebel) fighters systematically fire mortar and shoot into Russian territory which killed a Russian citizen," Lysenko said. (Reporting by Natalya Zinets; writing by Richard Balmforth, Editing by Timothy Heritage)