Russia has confirmed that it will attend talks with the European Union and Ukraine on Monday aimed at resolving a bitter gas supply dispute which threatens deliveries to Europe.

Russian state-owned gas giant Gazprom threatened this week to cut deliveries to Ukraine over a dispute related to Moscow's move to start direct supplies to eastern parts of the country controlled by pro-Kremlin rebels.

The EU said it has invited the Russian and Ukrainian energy ministers to Brussels on Monday for talks on resolving a bitter gas supply dispute which threatens deliveries to Europe.

Russia's state-owned gas giant Gazprom threatened on Tuesday to cut off deliveries to Ukraine, diverting deliveries instead to eastern parts of the country controlled by pro-Kremlin rebels.

U.S. Secretary of State John Kerry on Thursday welcomed his Serbian counterpart for talks on the conflict in Ukraine, and called for greater transparency in trying to implement a fragile ceasefire.

Serbia took over the presidency of the multinational body OSCE, which has hundreds of monitors in Ukraine trying to observe whether Russia, pro-Moscow rebels and the Kiev government are sticking by a second truce agreed earlier this month.

Pro-Russian forces in eastern Ukraine will likely wait until the spring to attack the port of Mariupol, US intelligence chief James Clapper predicted Thursday, adding such a move did not seem imminent.

Clapper said Russian President Vladimir Putin's aim was to establish a land bridge to Crimea, which was annexed by Moscow a year ago after the fall of a pro-Moscow leader in Kiev.

The frontline slicing across east Ukraine is more than a boundary fought over by the army and pro-Russian forces; it is also a partition many Ukrainian civilians in the separatist-held zone need to cross to live their lives.

And so, naturally, an underground business has sprung up in providing Ukrainian government passes -- one in which corruption is the byword.

The European Union called on Russia Thursday to "urgently" release Ukraine military pilot Nadia Savenchko on humanitarian grounds after 75 days on hunger strike to protest her detention.

A spokesperson for EU foreign affairs head Federica Mogherini said Russia "bears a responsibility for her very fragile health" after a Moscow court rejected Savchenko's appeal against her pre-trial detention.

Many NATO member countries are spending less on defense despite promises to boost their budgets in the face of growing tensions with Russia, a think-tank said Thursday.

The European Leadership Network looked at figures for 2015 from 14 NATO members to see if they meet pledges made at a summit of the U.S.-led military alliance last September.

Ukraine's military on Thursday said it had started the withdrawal of heavy weapons from the frontline, bolstering a stuttering peace plan as the United States and Russia traded barbs over the conflict.

The announced pull-back -- a key part of a peace deal negotiated earlier this month -- comes after a shaky truce that was meant to come into force on February 15 but only took hold across the conflict zone in recent days.

Pensioner Anatoliy gestures at a row of burnt out cars as he worries about what might be in store for his east Ukraine hometown -- the frontline port city of Mariupol.

"People are afraid and don't want this to be repeated," he tells AFP.

Ukraine's army awaited orders to pull back weapons from the frontline Thursday as a shaky truce settled, while the United States and Russia traded blame for delays in the peace plan.  

Kiev said that its positions around the conflict zone came under fire from pro-Russian rebels a relatively low seven times in 24 hours, and neither side reported any fatalities for the second day running.