Aleksandr Zakharchenko told the BBC he wanted to expand the self-proclaimed Donetsk People's Republic (DNR).
He said the truce agreement brokered by the West in Minsk in February was not being properly implemented by Kiev.
"Ukraine doesn't want to resolve all the issues," he said. Kiev has repeatedly denied the claim.
In other developments:
"If you agree to resolve something, then you need to act and move forward, and resolve everything that's included," said Mr Zakharchenko, the DNR head.
"If that doesn't happen, then the Minsk agreement is unfulfilled, and it renders all the meetings in Minsk pointless."
He also accused Kiev of preparing for war - a charge Ukraine denies.
Under the Minsk agreement, backed by France, Germany and Russia, Ukraine's government claims that the rebel-held east will remain part of the country.
But Mr Zakharchenko insisted it must be legally recognised as an independent territory.
He said: "Ukraine has stopped paying welfare, pensions and other payments that are obligatory for a state to pay its citizens.
"They don't do it, so they've de facto recognised us."
The BBC's Tom Burridge reports that throughout this week shelling could be heard in central Donetsk, a sign that the Minsk deal had not brought real peace to the region.
On Saturday, Ukraine said the rebels had attacked government positions in the eastern Luhansk region - but the assault was repelled.
Ukraine and the rebels both claim to have withdrawn heavy weapons from the line of contact.
The UN says at least 6,116 people have been killed since the fighting began in the Donetsk and Luhansk regions last April - a month after Russia annexed the Crimean peninsula.
Ukraine accuses Russia of arming the rebels and sending Russian troops over the border - a claim Moscow denies.
A senior separatist leader in eastern Ukraine has claimed an agreed ceasefire deal will fail unless Kiev recognises the independence of rebel-held areas.