The vice president of the European Commission, Maroš Šefčovič, has said he has seen no move at all from the UK side in negotiations on changes to the Northern Ireland Protocol.
He urged the British government to engage with the EU "sincerely".
Speaking after the third round of talks on the protocol, Mr Šefčovič said he found the lack of movement from the British side to be "disappointing".
He also warned the UK that triggering Article 16 of the protocol would have serious consequences.
He said the talks process will continue next week, and he will visit London next Friday.
However, Mr Šefčovič said he views next week as "an important one: we should focus all efforts on finding a solution as soon as possible". He said the aim should be to establish stability and predictability for Northern Ireland.
Mr Šefčovič said the package of changes to the protocol tabled by the Commission would "provide significant changes for operators on the ground", saying it would in effect create a type of "Express Lane" for imports from Britain that are destined only for the Northern Ireland market.
On the matter of Article 16 - the safeguard clause in the Northern Ireland Protocol - Mr Šefčovič said: "We hear a lot about Article 16 at the moment. Let there be no doubt that triggering Article 16 - to seek the renegotiation of the Protocol - would have serious consequences.
"Serious for Northern Ireland, as it would lead to instability and unpredictability. And serious also for EU-UK relations in general, as it would mean a rejection of EU efforts to find a consensual solution to the implementation of the Protocol."
Unusually the Commission vice-president made a formal statement on camera at the end of the meeting with Lord Frost, and expressed his frustrations in blunt language.
His British counterpart released a brief written statement, which said: "Lord Frost set out the UK's assessment of the negotiations on the Protocol. He underlined that progress had been limited and that the EU’s proposals did not currently deal effectively with the fundamental difficulties in the way the Protocol was operating. He added that, in the UK view, these gaps could still be bridged through further intensive discussions. He underlined that the UK’s preference was still to find a consensual solution that protected the Belfast (Good Friday) Agreement and the everyday lives of people in Northern Ireland."
The two sides also briefly discussed the UK-French fishing dispute over fishing licences for small French vessels in waters controlled by the Channel Islands.
Lord Frost told the Commission that the UK as a whole had licensed 98% of EU vessels seeking to fish in UK waters, representing almost 1,700 vessels, in line with its obligations under the Trade and Cooperation Agreement (TCA).
He repeated that vessels must provide the necessary evidence of historic fishing activity required by the TCA in order to receive a license.
Vice-President Šefčovič said: "The Trade and Cooperation Agreement is clear: vessels that were fishing in the territorial waters of the UK and the Crown Dependencies should be allowed to continue. All French vessels entitled to a licence should receive one. I support (fisheries) Commissioner Sinkevičius in his ongoing efforts to find a solution."
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Time 'running out' for deal on Northern Ireland Protocol
Earlier, Britain's Brexit minister warned that time was "running out" on talks with the European Union as the UK government threatened to suspend a deal over Northern Ireland.
"We hope to make some progress. But honestly, the gap between us is still quite significant. But let's see where we can get to," David Frost told journalists as he arrived for the latest round of negotiations in Brussels.
"Time is running out on these talks. If we're to make progress, we need to make progress soon."
The differences over Northern Ireland have embittered relations between Brussels and London and threaten to cause a trade war that could bring bilateral trade to a standstill.
Looming over the talks is Article 16 of the protocol, which gives either side the right to suspend parts of the trading arrangement in exceptional circumstances.
"We're not going to trigger Article 16 today, but Article 16 is very much on the table," Mr Frost said.
Britain has threatened to use the provision by early November if the EU does not redraw the protocol. Europe could retaliate a month later once it has done so.
Mr Frost insisted that the European Commission, the EU's executive arm, has to "listen" to demands to rewrite the protocol laid out by London if it wants to avert a breakdown.
The British government is pushing for major changes such as ending the oversight role of the European Court of Justice.
But Brussels refuses to renegotiate the protocol and has instead offered to ease customs checks on British goods entering Northern Ireland.
A European diplomat said the commission was increasingly readying itself for the possibility London could trigger Article 16 and warned of a "strong reaction" from the EU if that happened.
The diplomat said that might involve Brussels suspending the overall post-Brexit trade deal, a move that could plunge ties between the two sides and put them back to square one.
