Image copyright Getty Images Image caption Belarus is home to 30 million people and half of the country’s land mass is owned by Belarus
President Petro Poroshenko of Ukraine has asked the Belarus president to prevent border crossings being used by smugglers.
He’s asking Belarussian President Alexander Lukashenko to stop his government allowing smugglers to cross into Ukraine.
But Mr Lukashenko says that, however difficult, it’s not clear what Mr Poroshenko is talking about.
And the countries have had a history of tensions and conflicts. Here are some of the main issues involved.
Ukrainian border guards stopped about 12 Belarussian military officers crossing to Ukraine in January. The border guards said that they were “on instructions from Ukrainian authorities”, according to Belarusian news site TTK [Кобсема видала].
Image copyright Reuters Image caption Belarussian troops have occupied Donetsk airport, giving them some control over the city
In 2014, some Belarussian troops took over Donetsk airport, about 150km (90 miles) from the border. They later withdrew.
More border incidents came in 2014 and 2015, and Ukraine’s border guards sometimes detained Belarussian soldiers and officers crossing the border.
Relations between the two countries hit a new low in August 2015 when Ukrainian officials accused Belarussian troops of occupying Slovyansk, one of two towns in the east of Ukraine that has been occupied by separatists for several years.
Ukraine also accused Russia of war crimes in Donbass, the eastern region.
Mr Lukashenko has accused Ukrainian President Petro Poroshenko of “an aggressive policy against his own country” after the latter said he could not be sure that a truce offered by Russian President Vladimir Putin was binding.
Image copyright Bloomberg Image caption Mr Lukashenko has ruled Belarus since 1994
The leaders met in July 2014 after months of violent clashes in eastern Ukraine, in which more than 9,000 people died, and agreed on a cease-fire. The deal was more or less observed.
Mr Lukashenko has accused Mr Poroshenko of trying to weaken Belarus economically by bringing in large numbers of Ukrainians to live in Belarus.
“The conflict resolution in Donbass, at present limited, involves a number of constructive and common positions that the leaders of the Ukraine-Russia-Belarus dialogue in their common characteristic, are ready to reach,” Mr Lukashenko told reporters.
Image copyright AFP Image caption Petro Poroshenko has asked Belarus President Alexander Lukashenko to stop border crossings being used by smugglers
But relations have deteriorated in the past two years. Belarus blamed Ukraine for war crimes.
Mr Poroshenko, who has been in power since 2003, has become more outspoken about Russia.
Image copyright Reuters Image caption The atmosphere between the two leaders has deteriorated dramatically in the past two years
He suggested in June that more military steps would be taken in Donbass and described Russia as a “Great Satan”.
Mr Lukashenko, who has ruled Belarus since 1994, does not seem to have any intention of stepping down. He won the 2014 presidential election with 78% of the vote.
“A complex structure of mechanisms for public freedom on Belarusian territory is being drawn up. It should become visible after the Perestroika era in 1990,” he said in a speech in December.