KIEV, July 14 - Ukraine accused Russian army officers on Monday of fighting alongside separatists in the east of the country and said Moscow was once more building up its troops on the joint border, Reuters reported.

A missile that downed a Ukrainian transport plane carrying eight people near the border was probably fired from Russia, Ukrainian officials said.

President Petro Poroshenko held 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 intense fighting, in which Ukrainian forces say they inflicted heavy losses on the rebels, marked a sharp escalation in the 3-1/2 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.

Diplomatic efforts to defuse the crisis have so far made little progress. However, German Foreign Minister Frank-Walter Steinmeier said a "Contact Group" - which includes Russia, Ukraine and the Organization for Security and Cooperation in Europe (OSCE) - aimed to talk to the rebels by video link on Tuesday and meet them in person soon afterwards.

Speaking after what he described as "difficult talks" on the telephone with his French, Russian and Ukrainian counterparts, Steinmeier said all parties were making a "strong effort" for the Contact Group to hold the video conference and agree a venue for the direct meeting.

Poroshenko made similar complaints of Russian incursions on Sunday to the European Union with an eye to pushing the bloc to exert greater pressure, and possibly more sanctions, on Moscow.

Poroshenko told his security chiefs that government forces, which lost 23 men in a rocket attack on an army camp last Friday, were now facing a new Russian missile system and there would have to be a change in tactics. He gave no details.

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."

NATO said Russia had increased its forces along the border and now had 10,000-12,000 troops in the area.

Lysenko added that three Ukrainian soldiers had been killed and 12 more injured in the fighting in the past 24 hours.

Moscow's response to the cross-border shelling and the Ukrainian reports of Russian troops being moved up to the border raised again the prospect of Russian intervention, after weeks in which President Vladimir Putin had appeared intent on disengaging, pulling back tens of thousands of troops he had massed at the frontier.