Russia has warned that it could block international flights through its airspace if the EU goes ahead with new sanctions over the Ukraine conflict.
The EU is expected to announce shortly whether further sanctions will take effect now or be put on hold.
The EU has said this will depend on how the situation develops on the ground.
Pro-Russian separatists have recently made big gains, but a fragile ceasefire in eastern Ukraine appears to be holding despite some sporadic shooting.
Fighting in the east has killed some 2,600 people since April. The truce and roadmap to peace were agreed on Friday.
Russia has repeatedly denied accusations by Ukraine and the West that it has been sending troops into Donetsk and Luhansk regions to help the rebels, who want to establish an independent state.
Russian Prime Minister Dmitry Medvedev warned that Moscow would respond "asymmetrically" to further sanctions.
A Russian airspace ban "could drive many struggling airlines into bankruptcy", he told a Russian daily according to BBC.
"If there are sanctions related to the energy sector, or further restrictions on Russia's financial sector, we will have to respond asymmetrically... For example, restrictions in the transport sector.
"We work on the basis of friendly relations with our partners, and that's why Russia's skies are open to flights. But if we are restricted then we'll have to respond," he told Vedomosti (in Russian).
Airlines would have to pay far more for fuel if Russia blocked their routes to Asian destinations, and flight times would be longer in many cases.
Last week an EU official told the BBC that further sanctions would deepen the existing measures, affecting Russia's access to capital markets, dual-use goods which can be used for military purposes, defense equipment and some other sensitive technologies.
They would also expand the visa bans and asset freezes on Russian officials and entities, including separatist leaders in Ukraine.
The Ukrainian authorities in Donetsk region say President Petro Poroshenko is expected to visit Mariupol on Monday.
It is the last city in Donetsk region still held by the Ukrainian government and some shelling was reported there at the weekend. It is a strategic port on the route to Crimea, the peninsula annexed by Russia in March.
There was also some fresh shelling near Donetsk airport. The rebels are still holding the city, and have pushed back Ukrainian forces on the outskirts.
On Sunday, Ukrainian security official Volodymyr Poliovyi said 864 Ukrainian soldiers had been killed since the conflict began.
So far there have been no big prisoner exchanges since the ceasefire took effect.
EU 'to Review Russia Sanctions If Ceasefire Holds'
Brussels is "ready to review" new sanctions it plans to impose on Russia next week if a truce between pro-Moscow rebels and Kiev holds, European Council President Herman Van Rompuy said on Sunday.
"We have noted that Russia only consented with difficulty to serious negotiations. The ceasefire is an import step, but it is only a step," Van Rompuy told Belgian television channel VRT.
"We are ready if the ceasefire is durable, and/or if the peace talks start, to review these sanctions."
The new round of sanctions, agreed on Friday, would tighten existing measures imposed in July, targeting more individuals with travel bans and asset freezes, as well as tightening access to capital markets for Russian oil and defence companies.
They will be formally approved on Monday, although the full details of the people and organizations affected will not be released for another day.
EU leaders, along with US President Barack Obama, agreed to impose the sanctions despite a truce signed on Friday between pro-Moscow rebels and Kiev to end fighting in eastern Ukraine.
On the ground, the agreement was already looking shaky on Sunday after fighting broke out around two flashpoint cities overnight, killing one woman and sparking renewed fear among residents.
"The most important thing was to achieve a ceasefire. There have been a few incidents. We have to wait a bit to see if these incidents will continue or if they are exceptions," AFP quoted Van Rompuy as saying.
"There is a ceasefire after nearly 2 000 have died, do not forget. Between that and peace, there is still a big step and negotiations should begin as soon possible," he added.
According to AFP calculations, the five-month conflict in Ukraine has claimed 2 800 lives and sent at least half a million fleeing their homes.
Russia said on Saturday that it "react" if the EU imposes new sanctions.
"Russia's economy is suffering. They are already in recession, mainly because of the sanctions," said Van Rompuy.
