Ukraine's cease-fire is barely staying intact as separatists continue their attacks.

The pro-Russian separatists have shelled army positions in Ukraine and have attempted to storm the Donetsk airport, according to the Kiev government. The cease-fire "curtailed casualties for four weeks" but is now "starting to fray," Bloomberg reports.

Fighting in Donetsk, the largest city in Ukraine's conflict zone, continued Tuesday, according to a government statement. The government reported shelling at the Ukraine-Russia border that resulted in power loss for a number of districts. The attacks are in stark contrast with President Petro Poroshenko's statement last week that "the worst of the war is over" as Ukraine prepares for its elections, Bloomberg notes.

Meanwhile, U.S. Ambassador to Ukraine Geoffrey Pyatt reports that Russian forces and equipment are still in Ukraine, and the Russia-Ukraine border has not been secured. Pyatt tweeted that Russia and the pro-Russian separatists should adhere to the cease-fire. The U.N. estimates that more than 3,500 people have been killed in the current Ukraine conflict.