The attacks occurred in the cities of Donetsk and Luhansk.
The cease-fire extends to next Friday, but Ukraine's president said his troops will retaliate if fired upon.
Poroshenko is under pressure to secure support from Russia, as well as his backers in the West.
Ukrainian officials claimed that 300 pro-Moscow separatists were reported killed in clashes in the country's restive east.
The new fighting on Friday was about 100 km (60 miles) from the border with Russia.
Ukraine's president has announced plans to implement a unilateral cease-fire.
American space companies said they still have contracts for Russian equipment, but they're rushing to develop their own technology.
Poroshenko was due later on Thursday to meet regional officials from the Donetsk and Luhansk areas of eastern Ukraine.
Biden's warning came as fighting continued in the east, which has killed 147 Ukrainian soldiers and wounded hundreds more.
Maksim Chmerkovskiy might leave "Dancing with the Stars" after he said he wants Meryl Davis to be his last partner.
Two Russian state-television journalists were killed in shelling near the eastern city of Luhansk.
Imports of hot-rolled, flat-rolled, carbon-quality steel products from Russia rose 162 percent in the three months through April, compared with the previous three-month period, Reuters reported in an exclusive article.
The Interior Ministry issued a statement that described the blast as "the latest attempt by the Russian side to discredit Ukraine as a partner in the gas sector."
Google says it will use Skybox's satellites for Maps, but the company's CEO paints a more intriguing picture of corporate espionage.
Vitaly Churkin, Russia’s ambassador to the U.N., claimed that nearly 60,000 Ukrainian refugees are now in Russia.
The young defender scored the game winner for the U.S. over nemesis Ghana.
Gazprom shut off natural gas to Ukraine on Monday, leaving the country relying on a less-than-half-full inventory.
Exxon's CEO will speak at a summit in Moscow despite requests from the U.S. government to isolate Russia over its actions in Ukraine.
Supply of Russian gas to Europe will continue through Ukraine as usual unless the latter diverts it during transit.
Russia incensed over statements by acting Ukranian foreign minister describing Putin in unflattering terms.
The aerial images reportedly show Russian T-64 tanks moving near the Ukrainian border.
The protest comes after eastern separatists downed a Ukrainian military plane.