BRUSSELS - The European Union geared up a fresh wave of sanctions against Russia on Saturday with warnings that the escalating conflict in Ukraine was putting all of Europe at risk of war.
Fears of a wider confrontation spiralled after allegations that Russia has sent troops and weapons across the border to help a bloody new counter-offensive by pro-Kremlin rebels.
Ukrainian President Petro Poroshenko visited Brussels to plead his case for a firm response, before the EU's 28 leaders were to hold talks on the worsening situation.
The latest rebel push in a conflict that has claimed nearly 2,600 lives has wrested several key towns in southeastern Ukraine from Kiev's control in recent days.
EU Commission head Jose Manuel Barroso warned that the crisis was near a "point of no return" and said Brussels had drawn up tougher sanctions against the Kremlin.
Poroshenko said Ukraine was the victim of "foreign military aggression and terror" and alleged that thousands of Russian troops and hundreds of tanks were on Ukrainian soil.
"Today we are talking about the fate of Ukraine, tomorrow it could be for all Europe," he warned.
French President Francois Hollande said the EU would "no doubt increase" sanctions, while British Prime Minister David Cameron said the lessons of history demanded action.
Russia 'in war with Europe
But Lithuania's President Dalia Grybauskaite, whose Baltic nation is wary of a resurgent Russia on its own borders, said the EU should go further and send military equipment to Kiev.
"Russia is at war against Ukraine and that is against a country which wants to be part of Europe," she said.
"Russia is practically in a state of war against Europe."
The EU and US have already slapped tough sanctions on Russia over its role in the Ukraine crisis, including Moscow's annexation of Crimea in March.
New measures were likely to focus on extending current sanctions covering financial services, armaments, dual-use products and energy, Finland's Prime Minister Alexander Stubb told reporters.
Brussels could deliver a further riposte to Moscow if, as expected, it names Polish premier and vocal Kremlin critic Donald Tusk as new EU president.
NATO said Thursday that Russia had sent at least 1,000 troops to fight alongside the insurgents, along with air defence systems, artillery, tanks and armoured vehicles, and had massed 20,000 troops near the border.
The fresh rebel offensive has raised fears the Kremlin could be seeking to create a corridor between Russia and the strategic Black Sea peninsula of Crimea which Moscow annexed in March.
Moscow has denied any troop presence in its western neighbour, despite the capture of paratroopers by Kiev and reports of secret military funerals being held in Russia.
Plane shot down
Ukraine has openly asked the EU for military help, and on Friday it announced that it would also seek membership of the NATO alliance, a move sure to further enrage the Kremlin.
Poroshenko will travel to the NATO summit in Wales next week to meet US President Barack Obama and seek practical help from the Western alliance.
The sudden surge in tensions came only days after Putin and Poroshenko held talks which failed to achieve any breakthrough.
On the ground there seemed to be little let up for Ukrainian government forces, who have been fighting the separatists in the industrial east since April and had recently been boasting of major advances.
Kiev said Saturday that another airforce plane has been shot down in the east, blaming it on a "Russian anti-aircraft system".
Faced with the reinvigorated insurgent push that has dramatically turned the tide of the conflict, Ukrainian forces have been trapped in a string of towns in the southeast.
Kiev's contingents began a withdrawal from besieged positions near the transport hub of Ilovaysk, which lies east of the main rebel stronghold of Donetsk, after holding ground without reinforcements for 10 days.
Pro-Russian militants in Donetsk boasted on Friday that the insurgency now has full control of the border with Russia.
In the Azov Sea port city of Mariupol to the south of Donetsk, citizens dug trenches as they geared up to defend the city from a feared rebel offensive from the east.
The nearby town of Novoazovsk was captured Wednesday by the separatists, who Kiev says were substantially helped by Russian troops.
"We are Ukrainians, we are not slaves," Mariupol resident Alexander, a shoe salesman, told AFP. "Help us! Give us weapons and we will do the rest," he said.
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EU readies new Russia sanctions as Ukraine conflict spirals