KIEV, Ukraine (CNN) -

Three people were killed and four others were injured Saturday when mortars were fired during an attack on a Ukrainian military base in Slovyansk by pro-Russia separatists, the Ukrainian Counter-Terrorist Operation office said Saturday.

The attack comes after a cease-fire between Ukrainian government forces and separatists was extended through 10 p.m. local time Monday, according to Ukrainian President Petro Poroshenko's office.

The cease-fire extension suggested that Ukraine may be moving back from the brink of full-fledged civil war. However, the situation remains volatile, with continued violence and the constant fear of more unrest, cease-fire or not.

The crisis has its roots in former President Viktor Yanukovych's decision to shun a European Union Association Agreement last year and work with Russia instead. The move unleashed deadly strife that led to Yanukovych's ouster, Ukraine's loss of Crimea and a pro-Russia separatist rebellion.

Poroshenko seeks a commitment to negotiations with Russian separatists, the release of hostages, a mutual cease-fire and the withdrawal of Russian-backed forces from Ukraine. He has insisted that the Russian annexation of Crimea, which has an ethnic Russian majority, cannot stand.

Russia and Ukraine have been engaged in a tense standoff since the annexation in March when Russia also massed troops along its western border with Ukraine.

Poroshenko's office announced Friday a 72-hour extension of a fragile cease-fire that was declared last week.

That means that, by late Monday night, the Kiev government expects that all hostages will be released, all international border checkpoints will be taken out of separatists' hands and the militants' recruiting offices in Russia will close down.

At the same time, the Ukrainian government says it still reserves the right to use force in any areas where the cease-fire is not being universally accepted.

Separatists released four international observers being held in captivity for a month in eastern Ukraine, said the Organization for Co-operation in Europe via Twitter on Saturday. That followed Thursday's release of four other OSCE observers, according to the organization.

Yet, Saturday's attack on the Ukrainian military base in Slovyansk by pro-Russia separatists was just the latest example of violence since the original cease-fire was declared.

On Thursday, a Ukrainian national guard base came under attack in Donetsk, Anti-Terrorist Operation unit spokesman Vladislav Seleznyov said. Ukrainian troops were able to repel the attack but braced for another assault.