Lorries from a Russian convoy carrying aid to eastern Ukraine have reached a border post controlled by separatists.
But they seem unlikely to cross into Ukraine immediately as the Red Cross said it had still not received security guarantees for the convoy to continue.
Earlier Ukraine's military said that separatists had shot down a government fighter jet near the rebel-held city of Luhansk in the east of the country.
A military spokesman said the pilot had ejected and was safe.
In other developments:
- Ukrainian Security Council spokesman Andriy Lysenko says government forces have taken a rebel-controlled police station in Luhansk and raised the Ukrainian flag "after a battle for... a residential district". If confirmed, this would be the first time government forces have entered the city since the rebels seized control
- Rebel sources say 10 civilians had been killed in the city of Donetsk over the past 24 hours as government forces continue an offensive
- Mr Lysenko says a convoy of rocket launchers has been seen crossing from Russia into Ukraine, days after two Western journalists reported seeing military hardware entering from Russia. Russia has denied sending any weapons
- German Foreign Minister Frank-Walter Steinmeier says a new political impetus is needed to resolve the crisis and to avoid further escalation
- The Russian, Ukrainian, German and French foreign ministers are scheduled to hold talks on the eastern Ukraine crisis in Berlin later on Sunday
More than 2,000 civilians and combatants have been killed since mid-April, when Ukraine's government sent troops to put down an uprising by pro-Russian separatists in the Donetsk and Luhansk regions.
But it faced a new challenge on Sunday as the leader of the ultra-nationalist Right Sector threatened to withdraw volunteers fighting on the government side.
Dmytro Yarosh said Right Sector would launch a "campaign in Kiev" if its demands, including the release of detained members, were not met within 48 hours.
He called on President Petro Poroshenko to "immediately bring order" to the Interior Ministry, which he accused of harbouring "revanchist forces".
Analysis: David Stern, BBC News, Kiev
Ukraine's government seems to have made a devil's bargain, by allowing the formation of loosely-controlled paramilitary groups to fight in the country's east.
That some of these volunteers are ultranationalists, far-right extremists and sometimes even neo-Nazis seemed to be worth the gamble. Anyone who was willing to risk his life to defend Ukraine at this critical juncture was welcome.
Also figuring into their logic was the fact that the insurgency itself is a more pressing threat than far-right fighters, that these individuals are a minority among the Ukrainian forces and that Nazis and extremists are heavily represented among the pro-Russian militants.
Still, there has been virtually no public debate about whether it was a good idea to give guns to people and groups with extremist views. Now, with Right Sector threatening to march on the capital to enforce their demands, maybe this very overdue discussion will begin.
The government plane had been shot down after launching an attack "to eliminate a large group of rebels", Ukrainian military spokesman Leonid Matyukhin said, quoted by AFP news agency.
The pilot ejected from the aircraft safely and rescuers delivered him to a safe location, the spokesman added.
Meanwhile some 16 vehicles from the 280-lorry Russian convoy carrying humanitarian aid for eastern Ukraine were seen arriving at the border.
The Red Cross, quoted by Reuters news agency, said Ukrainian and Russian customs officials had agreed to inspect the lorries.
But the Izvaryne crossing where the vehicles have arrived is controlled by rebels, so it is not clear how Ukrainian officials will reach them.
Kiev has insisted that any aid sent to eastern Ukraine from Russia should cross a government-controlled part of the border.
Also, Ukraine's Security Council tweeted that its border guards had not received any paperwork for the cargo.
The convoy has been parked near the town of Kamensk-Shakhtinsky for several days after setting out from near Moscow on Tuesday, said to be carrying 2,000 tonnes of aid.
Russian officials quoted by Russia's Ria news agency said that lorries were being sent out in small groups to avoid causing traffic jams, but there were no plans for any of them to cross the border on Sunday.
The Ukrainian government said late on Saturday that it had declared the convoy "legal", but Red Cross officials speaking at the time said it had still not been given clearance because of some outstanding security issues.
There had been fears expressed by Ukraine and by Western governments that the convoy could be carrying arms for the rebels or could be used as a pretext by Russia for military action.
Russia has denied any military involvement with the convoy.