The UK and EU "should not pretend to be negotiating" a Brexit deal if there are no new proposals on the table, EU chief negotiator Michel Barnier has said.
He said the UK telling the EU what it does not like was "not enough".
He cast doubt on a UK proposal to give Northern Ireland a future veto over EU rules, saying all parts of the UK would have to sign up to the terms of exit.
The government said it had offered "a number of proposals" as alternatives to the Irish border backstop.
The backstop - an insurance policy designed to avoid a hard border on the island of Ireland - has proved a key sticking point.
The government said "constructive discussions" were ongoing and the UK had been "clear" in those discussions "that the antidemocratic backstop needs to be removed" if a new deal was to be reached.
Boris Johnson has insisted a deal is possible at a crucial summit of EU leaders on 17 October - although ministers have been reluctant to reveal the details of new proposals in advance for fear they will be "rubbished" by the EU.
The PM has insisted he will not accept a further delay beyond 31 October despite MPs passing a law requiring him to seek an extension if there is no deal by 19 October.
After meeting Mr Barnier and Mr Juncker in Luxembourg on Monday, Mr Johnson said both sides agreed to accelerate efforts to reach an agreement.
There were significant moments in Strasbourg this morning, even if the discussion lacked the fireworks present in the Brexit debate elsewhere.
Jean-Claude Juncker clearly signalled that in his last few weeks in office he will show solidarity with the Republic of Ireland rather than siding with the UK to get a deal. That will disappoint those in the UK who bank on him wanting an agreement to secure his legacy.
The EU chief negotiator, Michel Barnier, gave a cautious thumbs-up to the big British idea of an all-Ireland zone for plant and animal health.
But he appeared to give a thumbs-down to another - giving the Stormont Assembly a decisive say over the Irish backstop, or whatever takes its place.
Briefing the European Parliament, Mr Juncker said the lunch had been "friendly and constructive" but there had been no progress on the main sticking point - the UK's demand that the Northern Irish backstop should be removed from the current agreement.
Mr Juncker said any alternative to the backstop must achieve the same objectives - to prevent the need for physical infrastructure on the border with the Republic of Ireland, to safeguard the EU's single market and protect all-Ireland economic co-operation.
"I said to Mr Johnson that I have no emotional attachment to the backstop but I stand by the objectives it is intended to achieve," he said.
"That is why I called on the PM to come forward with operational proposals in writing.
"Until such time those proposals have been presented, I will not be able to tell you looking you straight in the eye that any real progress has been achieved."
Mr Barnier said the UK had made it clear which parts of the backstop - which would see Northern Ireland closely tied to the single market and the UK follow EU customs rules until a new trade deal is agreed - it did not like, but "that is not enough to move towards a solution".
Main messages from @JunckerEU & @MichelBarnier in Strasbourg: EU won't ditch Ireland, @BorisJohnson plan for "Stormont lock" is not acceptable, reducing level playing field commitments for future relationship is bad idea, customs union is still available to Johnson's opponents.— Adam Fleming (@adamfleming) September 18, 2019
"Almost three years after the British referendum, ladies and gentlemen, it is certainly not a question of pretending to negotiate," he said.
If the UK wanted to remove the backstop, he said it must come up with answers to all the problems the temporary "safety net" was designed to solve.
However, he appeared to reject UK proposals to give the Stormont Assembly in Belfast a say over how much Northern Ireland conforms with EU customs rules and diverges from England, Wales and Scotland while the UK remained in any backstop arrangement.
"It is up to the UK government to ensure the support of the Northern Irish institutions for the withdrawal agreement that would be signed on behalf of the whole of the UK," he said.
Following a three-hour debate, the European Parliament approved a motion calling for any Brexit deal to include a backstop and also voted for the UK to be granted a further extension beyond 31 October if it asks for one.
During the session, MEP Guy Verhofstadt called on the UK to give all three million EU nationals living in the country an automatic right to remain.
Rather than channelling the "angry Hulk" - a reference to Mr Johnson's recent comparison of the UK to the Incredible Hulk - the Parliament's Brexit spokesman said the PM should adopt the persona of a "caring nanny", such as Mrs Doubtfire.
But Brexit Party leader Nigel Farage said it was clear the UK and EU were paving the way for an agreement next month which would be portrayed as a "victory" for both sides.
Even without the backstop, he said the deal on the table would be "bad" for the UK as it would see it "trapped in EU rules and under the auspices of the European Court".
He also criticised they way Mr Johnson was treated during a visit to Luxembourg last week.
He said the country's "pipsqueak" leader Xavier Bettel had "ritually humiliated" his counterpart by appearing at a press conference without him and berating his Brexit policy.