Kiev (AFP) - President Petro Poroshenko warned Sunday that Ukraine must be ready to defend itself should a peace plan aimed at ending a five-month war with pro-Russian insurgents fall through.

The pro-Western leader's comments followed a warning by a top defence spokesman that Ukraine would not pull back its forces from the front line as required under a new truce until all sides put down their guns.

Poroshenko delivered an 80-minute televised defence of his high-stakes decision Tuesday to hand a wedge of the industrial east three years of effective autonomy in return for the rebel-held area remaining a part of Ukraine.

A September 5 ceasefire was reinforced on Saturday in a European-mediated deal signed with separatist leaders and Russia that required all sides to halt fire within 24 hours and set up a 30-kilometre (20-mile) demilitarisation zone.

Poroshenko had been accused of essentially admitting defeat to the Kremlin by nationalist politicians who are jostling for votes ahead of parliamentary polls at the end of next month.

And the 48-year-old chocolate baron came out swinging in his first extended question-and-answer session with reporters since the day after his May presidential election win.

"We must be ready to protect our country if the peace plan does not work," he said.

"Believe me, we have the means to defend ourselves," he added in reference to supplies of radar and other non-lethal military equipment he secured during his visit to Washington on Thursday and talks with NATO allies.

Poroshenko conceded that the advanced technology was especially badly needed because 65 percent of the military hardware -- much of it left over from the Soviet era -- sent into the war zone had been destroyed.

National Defence and Security Council spokesman Andriy Lysenko said however that Ukraine would not withdraw its forces to allow the creation of a buffer zone until the rebels stopped shooting.

"If (Ukrainian forces) are withdrawn, it will be done simultaneously with the Russian troop withdrawal."

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NATO's top military commander had warned Saturday that Russian forces were "still in Ukraine" -- a menace that meant that the truce on the ground was holding "in name only".

But Poroshenko argued that the conflict "cannot be won by military means alone" and stressed that the truce was reached thanks to three "very long" discussions with Russian President Vladimir Putin.

- Protest at 'Putin's war' -

The interview was aired only hours after thousands of Russians came out in Moscow for their first major protest march against the Kremlin's involvement in Ukraine since the separatist uprising erupted in April.

Some carried placards reading "forgive us Ukraine" while other demanded "jail for Putin" in chants that rang out through a crowd organisers put in the tens of thousands.

"I believe the war has been provoked by Putin," said wheelchair-bound protester Vladimir Kashitsyn.

Putin himself has stayed mum since the signing of Saturday's peace memorandum in the Belarussian capital Minsk and the Moscow march made barely a mention in the state media.

But Kremlin chief of staff Sergei Ivanov told Monday's edition of the Rossiyskaya Gazeta government daily that Poroshenko "has started to realise he does not need a war to the winning end -- in other words, until there are no more Ukrainians left standing".

The level of violence across industrial eastern Ukraine appeared to have dropped off Sunday after an overnight spurt of heavy gunfire in the separatist stronghold of Donetsk.

Kiev said two of its soldiers had died in sporadic clashes that broke out in the hours since Saturday's nine-point memorandum was signed.

- OSCE monitors -

The deal is meant to reinforce the September 5 truce forged in a bid to stem fighting that has claimed nearly 3,000 lives since April and threatened Ukraine's very survival.

The UN office for humanitarian affairs said the conflict had inflicted damaged worth 340 million euros ($440 million) in the predominantly Russian-speaking war zone.

The OSCE pan-European security body that mediated the peace talks said it had 80 observers in the war zone ready to monitor the agreement's implementation.

"De-escalation -- including the silencing and withdrawal of weapons and clearing of mines and unexploded ordnance -- is a vital first step," it said in a statement.

The Minsk pact came together only after all sides agreed to leave the most divisive political issues over the status of the rebel-held areas in Ukraine's rustbelt for future negotiation.