The Ukrainian military says five of its soldiers died in heavy rebel shelling of Ukrainian army positions at Avdiivka just north of rebel-held Donetsk.
Three Ukrainian troops died earlier when a mine blew up, the military said.
The US Department of State called it "the deadliest one-day period in 2017" in the eastern Ukraine conflict.
In a video tweet on Thursday, the state department blamed the "Russian-led" rebels for the flare-up and urged both sides to observe the fragile ceasefire.
The rebels also reported clashes on Thursday but did not confirm the Ukrainian death toll.
The UN says more than 10,000 people have died since the eastern Ukraine conflict erupted in April 2014, soon after Russia annexed Ukraine's Crimea peninsula. The fighting has displaced more than 1.6 million people.
A ceasefire was agreed in Minsk in February 2015, but its terms are far from being fulfilled.
A Ukrainian military statement on Facebook said the pro-Russian rebels fired mortars, grenades and machine guns in the Avdiivka area, and Ukrainian troops returned fire.
It was the most intense fighting since 11 June when four Ukrainian soldiers were killed.
Western governments accuse Russia of helping the rebels with regular troops and heavy weapons. Moscow denies that, while admitting that Russian "volunteers" are helping the rebels.
On Tuesday the Donetsk rebel chief, Alexander Zakharchenko, announced plans to create a new "state" to replace Ukraine. He called it "Malorossiya" (Little Russia) - but the Kremlin later dismissed it as just his "personal initiative".