Britain's new Brexit plan got a cool reception Wednesday in Brussels, where European officials highlighted problems and their chief negotiator warned it left "a lot of work" to

be done. European Commission chief Jean-Claude Juncker used a call with Prime Minister Boris Johnson to share concerns about "problematic points" in the proposal, in particular London's proposed customs regime aimed at avoiding checks on the border between British-ruled Northern Ireland and EU member Ireland.

EU officials were careful not to dismiss the proposal out of hand, and pledged to continue talks, but there was widespread scepticism and concerns that the plan could leave Ireland exposed.

"There is progress, but to be frank a lot of work still needs to be done," Barnier said.

The proposal is Britain's suggestion for how to do away with the so-called "Irish backstop", a safety net clause which Johnson's predecessor Theresa May signed up to but which the British parliament rejected.

With time fast running out to agree and ratify a new deal before Britain's October 31 departure day, Juncker acknowledged "positive advances" in the British offer, telling Johnson EU negotiators would now "examine the legal text objectively".

"He acknowledged the positive advances, notably with regards to the full regulatory alignment for all goods and the control of goods entering Northern Ireland from Great Britain," Juncker's office said in a statement.

The backstop agreed by May would have kept Britain temporarily aligned to EU customs rules, but it has been rejected by British MPs three times.

Under the new proposal, Northern Ireland would follow EU regulations and standards for all goods, including food and agricultural products, but would leave the bloc's customs zone to stick with the rest of the UK.

But the EU -- which has consistently insisted the backstop is needed to protect both the balance of peace in Northern Ireland and the integrity of the bloc's single market -- still has reservations.

Guy Verhofstadt, the head of the European Parliament's Brexit steering group, said the initial assessment of "nearly every member" of the committee "was not positive at all" because the proposal does not offer enough safeguards to Ireland.

He suggested the UK offer was not a serious attempt at reaching a deal but an effort to shift blame for failure to Brussels -- a concern harboured by many in Brussels since Johnson took office vowing to take Britain out of the EU on October 31 "do or die".

- Customs worries -

The commission statement on the Juncker-Johnson conversation said the EU chief "also noted that there are still some problematic points that will need further work in the coming days, notably with regards to the governance of the backstop."

It added: "Another concern that needs to be addressed are the substantive customs rules."

Johnson's plan allows the Northern Irish assembly to decide whether to extend this arrangement once "every four years", putting a potential time limit on the plan that the EU has previously rejected.

Juncker reiterated the EU stance that any replacement for the backstop must achieve the same ends.

"He also stressed that we must have a legally operational solution that meets all the objectives of the backstop: preventing a hard border, preserving North-South cooperation and the all-island economy, and protecting the EU's Single Market and Ireland's place in it," the statement said.

After tough talk from Downing Street that Johnson's offer would be "take it or leave it", there was a measure of relief among some in Brussels that it appeared instead to be the first move in a new phase of negotiations.

But one EU diplomat reiterated Juncker's reservations about the customs arrangements.

"It doesn't even look from a British point of view that it's ready for immediate use," the diplomat said.

"There is still a wide gap to be bridged, but whether this will succeed will only become clear in the next few days."

EU and British negotiators will meet for further talks in the coming days, the commission said, adding that the European Parliament and European Council -- which groups member states -- would be informed "every step of the way".

The commission also stressed that Juncker would consult Irish Prime Minister Leo Varadkar "and listen carefully to his views".

In Dublin, Varadkar voiced doubts, saying London's proposals "do not fully meet the agreed objectives" for keeping the border invisible and free-flowing.