Ukraine vowed Tuesday to use all weapons at its disposal to withstand an alleged new pro-Russian rebel advance that added urgency to a goodwill visit by Britain's defence secretary.

Kiev accused the eastern separatist fighters of launching another missile assault north of Mariupol -- a government-held port that provides a land bridge between rebel territories and the Russian-occupied Crimea peninsula.

"We will use our entire arsenal and all the means at our disposal to beat back the enemy's assault," Ukrainian military spokesman Vladyslav Seleznyov told AFP.

"We cannot risk the lives of our soldiers," said the top Kiev army representative. "The rebels used to launch Grad missile attacks on a rare occasion. Now, it is an everyday occurence."

The militias accused Kiev's pro-Western forces of being responsible for the latest flareup along east Urkaine's 500-kilometre (300-mile) front.

A top insurgency leader accused Ukraine's army of firing 500 mortar shells and rockets at rebel positions since Monday afternoon.
He reported the death of one militia member and a civilian in the separatist Donetsk province.

"The Ukrainian armed forces have boosted their presence around Mariupol by another 1,000 servicemen," rebel commander Eduard Basurin told the separatists' official news site.

"They have sent up to 2,000 marines to reinforce their positions" in villages around the rebel stronghold of Donetsk.

Ukrainian President Petro Poroshenko said in May that Kiev's fighting contingent in the war zone included 50,000 soldiers and volunteers.

The rebels have not revealed the size of their fighting force. They have joined Moscow in firmly denying Poroshenko's claim that up to 9,000 Russian special forces are backing the separatists.

- Hot war -

The two self-proclaimed republics of Donetsk and Lugansk launched their armed uprising shortly after the February 2014 ouster of a Moscow-backed president in Kiev.

The clashes have killed more than 6,800 people and sent Moscow's relations with the West plunging to their lowest point since the Cold War.
The crisis has also left 1.4 million people homeless and sent Ukraine's economy -- heavily dependent on exports from the country's industrial east -- into a tailspin.

The West is still pinning hope on a February truce agreement that has often been ignored but which has also kept fighting limited to a handful of flashpoints.

British Defence Secretary Michael Fallon met Ukrainian Prime Minister Arseniy Yatsenyuk in Kiev on Tuesday before travelling to the nationalist west of the former Soviet country.

"We will continue to stand shoulder to shoulder with you in the face of Russian aggression, in the face of Russian-directed insurgency and in the face of Russian-inspired terrorism," Fallon said in comments dubbed over into Ukrainian by Kiev TV.

"The conflict has not settled into a frozen war: it is quite 'red hot'," he was further quoted as saying by Yatsenyuk's office.
Fallon is due to oversee how British military trainers are progressing in their efforts to train around 650 Ukrainian soldiers for modern warfare.

The United States and NATO members have redoubled their efforts to prepare the poorly-equipped force for a possible invasion from Russia -- a giant neighbour that denies any involvement in eastern Ukraine's 16-month revolt.

Russia has called such Western steps as confrontational and staged its own war games along the Ukrainian border throughout the year.