Ukraine's Foreign Ministry says NATO Secretary-General Anders Fogh Rasmussen will visit Kyiv, amid heightened tensions over allegations Moscow is massing troops along the Ukrainian border.
The ministry said in a statement that the August 7 visit, at the invitation of Ukrainian President Petro Poroshenko, is intended to discuss an upcoming meeting on the NATO-Ukraine partnership.
Earlier on August 6, NATO said a Russian force buildup near the Ukrainian border amounted to some 20,000 troops.
In a statement, NATO spokeswoman Oana Lungescu said Moscow could use the excuse of a humanitarian or peacekeeping mission to send them into Ukraine.
Also on August 6, Polish Prime Minister Donald Tusk said he had received information "in the last several hours" suggesting the threat of direct Russian military intervention in Ukraine is "higher than it was several days ago."
On August 4, Moscow announced what it said were military training exercises in central and western Russia, including all areas where Russia shares a border with Ukraine.
Those exercises include the mobilization of more than 100 warplanes and attack helicopters.
"We're not going to guess what's on Russia's mind, but we can see what Russia is doing on the ground -- and that is of great concern. Russia has amassed around 20,000 combat-ready troops on Ukraine's eastern border," NATO's Lungescu said.
WATCH: Former Polish dissident Adam Michnik has likened Russian President Vladimir Putin's actions regarding Ukraine to Adolf Hitler's annexation of the German-speaking part of Czechoslovakia in 1938. The veteran of Poland's anticommunist opposition told RFE/RL's Aleksei Dzikavitski in Warsaw that Europe should find ways to "stop Putin" despite potential economic costs.
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Adam Michnik: 'We Must Stop Putin'
That comes after United Nations officials said a humanitarian crisis is worsening in eastern Ukraine as fighting continues between Ukrainian government troops and pro-Russian separatists around the cities of Luhansk and Donetsk.
Heavy fighting was reported overnight within the city of Donetsk, including air strikes by Ukrainian planes on transportation infrastructure.
Ukrainian forces have effectively encircled the city, which is the headquarters for the separatists' self-proclaimed "Donetsk People's Republic."
Correspondents in Donetsk report that residents who have remained in the city were bracing for an expected ground assault.
A government spokesman in Kyiv said fierce clashes with pro-Russian rebels had left 18 soldiers dead over the past 24 hours. Andriy Lysenko, spokesman for Kyiv's National Security and Defense Council, said 54 soldiers had been injured.
The pro-Russian separatists now control less than 10 percent of the territory that makes up Ukraine's Donetsk Oblast, having lost large swaths of land to government forces during the last six weeks.
At the UN Security Council on August 5, Russia's UN Ambassador Vitaly Churkin said the humanitarian situation in the cities of Luhansk and Donetsk was "disastrous."
Churkin said Russia wanted to send a "humanitarian convoy" into those cities to help civilians there.
But some Security Council members blamed Russia for the crisis, alleging that separatists receive support from Russia and that the deployment of Russian troops on the border has emboldened the rebels.
Meanwhile, Dutch Prime Minister Mark Rutte said on August 6 that forensic work at the crash site of the Malaysian airliner in eastern Ukraine was being halted due to the worsening security situation in the area.
At a press conference in The Hague, Rutte said international investigators will return to the scene when it is safe.
The investigators from the Netherlands, Australia, and Malaysia are working to recover the remains of all 298 victims from Malaysia Airlines Flight 17, which was brought down in eastern Ukraine.
Western and Ukrainian officials say they have evidence that separatists shot the plane down with a Russian Buk antiaircraft missile.
Separatist officials initially bragged about shooting down a plane on July 17 just days after they'd boasted about obtaining Russian Buk missile launchers.
But after it became clear that a civilian passenger plane had crashed near Donetsk on July 17, separatist leaders denied any involvement in the incident and said they did not possess the Russian-built antiaircraft missile systems.
More than 220 coffins have already been returned to the Netherlands.