Ukraine's defence minister has accused pro-Russian rebels backed up by Moscow's forces of assembling a 40,000-strong army, sufficient for a "mid-sized European state".

Stepan Poltorak's estimates fly in the face of Russia's denials that it either backs the separatist fighters or covertly sneaks troops across its south-western border into the Ukrainian warzone.

Ukrainian president Petro Poroshenko last week put the number of active Russian soldiers in his former Soviet republic's industrial east at more than 9,000.

But Russian president Vladimir Putin described Russians fighting against Ukraine's pro-Western government troops as patriotic volunteers and off-duty soldiers who are answering "a call of the heart".

Moscow has also distanced itself from two Russians captured last month who told Ukrainian interrogators and Western reporters they were active members of the Kremlin's special military reconnaissance force.

The Ukrainian defence minister said the insurgent army, which is in control of parts of the Russian-speaking Lugansk and Donetsk regions that are home to about four million people, could muster more than 550 tanks and an arsenal of other heavy weapons.

"The combined size of the Russian armed forces and illegal armed formations... stands at more than 42,500," Mr Poltorak told a group of visiting lawmakers from NATO countries on Monday.

"This amount of weapons, as we all understand, would be sufficient for a mid-sized European state."

Mr Poroshenko said Kiev's own eastern force had grown to more than 50,000 after being limited to 5,000 soldiers at the onset of the war 14 months ago.

Ukraine's latest charges come against the backdrop of growing fears that the ceasefire forged in February with the help of Germany and France is now falling apart.

Clashes around the 10,000 population town of Marinka, which straddles the twisting line separating Kiev's forces from the militants, and other disputed villages claimed about 30 lives last week and threatened to escalate on Monday.

"Our positions near Marinka came under small weapons and mortar fire this morning," Ukrainian military spokesman Andriy Lysenko said.

A rebel defence spokesman denied the charges and called the situation around the heavily-damaged town "calm".

West hardening line on Russia

The international community had initially stopped short of formally accusing Mr Putin of sending troops into Ukraine in what appeared to be a bid to either break up the country, or stamp out its budding alliance with the European Union and NATO.

Washington and Brussels imposed their heaviest sanctions on Moscow in response to its March 2014 annexation of Crimea.

The eastern campaign saw those measures toughened but never saw Russia accused of an outright "invasion".

The International Committee of the Red Cross also initially labelled the conflict a "civil war".

Monitors from the Organization for Security and Co-operation in Europe (OSCE) have referred to "irregularly armed groups", but prefer to simply call the rebels by their self-proclaimed name - armies of the Donetsk and Lugansk People's Republics.

However, the West's tone appears to have recently hardened.

NATO commanders and some EU leaders have openly accused Mr Putin of sending out alarming war signals across eastern Europe.

US president Barack Obama used the G7 summit on Sunday to condemn "Russian aggression in Ukraine", and the US State Department last week noted the presence in Ukraine of "Russian-separatist" forces - a term it had avoided using for months.

The West's verbal assault has so far produced only a muted response from Moscow.

But, Ukraine's defence minister said he had recently found all previous communication lines with the Moscow military being gradually cut off.

"I personally have made several attempts to speak to the Russian defence minister, Mr (Sergei) Shoigu," Mr Poltorak said.

"But he always complained about a heavy workload and never agreed to talk."

AFP