France has raised the prospect of imposing tougher sanctions on Russia if pro-Russian rebels attack the port city of Mariupol in eastern Ukraine.
Kiev fears pro-Russian forces will target the city, which has been the subject of previous attacks in recent months.
Separatist rebels seized the town of Debaltseve in February. While Ukraine and Russia have since agreed a ceasefire, Paris is wary over the rebels' intentions on the city of 500,000 inhabitants.
"The problem today is particularly around Mariupol. We've told the Russians clearly that if there was a separatist attack in the direction of Mariupol things would be drastically altered, including in terms of sanctions," said French foreign minister Laurent Fabius.
"At a European level the question of sanctions would be asked again," Fabius told French radio on Tuesday.
The European Union, along with the United States, has imposed a range of sanctions on Russia over its role in the Ukraine conflict.
The allies accuse Russia of fuelling the fighting, providing firepower and manpower to the pro-Russian rebels in eastern Ukraine.
In response, the West has slapped tough sanctions on the Russian economy, restricting Russian businesses' access to global financial markets
Ukrainian military and pro-Russian rebels agreed to withdraw heavy weapons from the front lines of the conflict and effectively establish a buffer zone on the ground. Both sides have accused each other of not respecting the ceasefire and violence has broken out on a number of occasions since Germany and France brokered a peace deal in Minsk earlier this month.
Ukrainian foreign minister Pavlo Klimkin said the spirit of the Minsk accord had been impacted by the rebels' seizure of Debaltseve and warned that Ukraine would respond to any attacks on its territory.
"Mariupol is critical," he said on Tuesday. "Any attack on Mariupol would change things...Debaltseve was a game changer because it disrupted the spirit of the Minsk accords."
The rebels have previously vowed to take control of the industrial city of Mariupol, stating that it is "part of Donetsk," the region which the rebels have claimed as self-declared republics.
If rebels seize the port, it would give them access to the sea, as well as a major psychological boost as the conflict approaches its second year.