Moscow (Alliance News) - Ukraine's parliament on Thursday passed a law to allow foreign military forces into the country as it seeks to quell a pro-Russian separatist uprising in its two eastern-most regions.

The legislation, which also outlaws any military forces acting against the government, seeks to specifically allow entry to peacekeepers from the United Nations or EU, according to a statement on the parliament's website.

This week has seen the deadliest fighting between separatist rebels and Ukraine's military since a ceasefire was signed in February.

The EU on Thursday condemned the flare-up as "the most serious violation of the ceasefire" that "risks unleashing a new spiral of violence and human suffering."

"The ceasefire must be fully respected and all heavy weapons withdrawn," the EU's External Action Service said on its website.

The Ukrainian military said 80 separatist fighters were killed and 100 wounded in a unsuccessful attempt by the rebels to storm the government-held town of Marinka on Wednesday.

Five Ukrainian servicemen were killed in the fighting, Ukrainian military spokesman Andriy Lysenko told reporters in Kiev.

Both the Ukrainian government and the pro-Russian rebels have blamed each other for launching an attack around Marinka, 30 kilometres south-west of the rebel stronghold city of Donetsk.

European security monitors said that rebel forces were seen moving heavy artillery to the front line before the violence erupted.

A dozen tanks and other armoured vehicles were seen late Tuesday heading from Donetsk towards Marinka, the Organization for Security and Cooperation in Europe's (OSCE) monitoring mission said in a statement.

The ceasefire prohibits heavy artillery within 50 kilometres of the so-called "contact line."

The Ukrainian military informed the OSCE on Wednesday that it would move heavy artillery to the contact line to deal with the "real threat" posed by the fighting in Marinka, according to the OSCE statement.

The town was still under Ukrainian government control by Wednesday evening when the OSCE reported the situation as calm.

A Donetsk separatist group said Wednesday that 15 people, including civilians, were killed in the area it controls.

The separatist rebellion erupted last year after the country's former pro-Russian president, Viktor Yanukovych, was ousted amid mass protests in Kiev that called for closer ties with the EU.

More than 6,400 people have died in the conflict since April 2014, according to estimates by the United Nations.

Ukrainian President Petro Poroshenko told parliament on Thursday that there are more than 9,000 Russian troops supporting separatist rebels on Ukrainian soil.

Russia has backed the separatists' cause but has repeatedly denied that there are any active Russian troops in Ukraine.

Copyright dpa