British Labour Party leader Jeremy Corbyn rebuffed calls Sunday to denounce contacts with terror groups Hamas and Hezbollah, while declaring that his party is against anti-Semitism, amid a roiling scandal over accusations of widespread anti-Jewish and anti-Israel sentiments among Labourites.

Corbyn used a May Day rally to say the party “is absolutely against anti-Semitism in any form” after a tumultuous week that focused attention on the party’s attitude toward Jews instead of its campaigning efforts ahead of London’s mayoral race.

But as Labour attempted to push back against efforts to label it anti-Semitic, it also came under fire for Corbyn’s past contacts with Hamas and Hezbollah, both sworn to Israel’s destruction.

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A statement from Corbyn’s spokesperson said he would continue to engage such groups, while denying that doing so was tantamount to an endorsement.

“Jeremy Corbyn has been a longstanding supporter of Palestinian rights and the pursuit of peace and justice in the Middle East through dialogue and negotiation,” the statement read, according to the Telegraph newspaper. “He has met many people with whom he profoundly disagrees in order to promote peace and reconciliation processes, including in South Africa, Latin American, Ireland and the Middle East. He believes it is essential to speak to people with whom there is disagreement, particularly when they have large-scale support or democratic mandates.”

Earlier in the day, Israel’s newly installed ambassador to the UK Mark Regev slammed many of Britain’s self-proclaimed progressive politicians for “embracing Hamas.” Corbyn has himself talked of Hezbollah as his “friends” and encouraged dialogue with Hamas.

“You’ve had too many people on the progressive side of politics who have embraced Hamas and Hezbollah,” Regev told the BBC’s Andrew Marr. “Both of them are anti-Semitic organizations; you just have to read Hamas’s charter and it’s like chapters straight out of the Protocols of the Elders of Zion. Yet some progressive politicians have embraced Hamas.

“Now, I’d ask the following question: If you’re progressive, you’re embracing an organization which is homophobic, which is misogynistic, which is openly anti-Semitic. What’s progressive about that?”

The party is also being criticized because of Corbyn’s past links to Interpal, a controversial British charity said by US officials to be backing extremist causes.

In April, Interpal helped sponsor a festival in Gaza in which students presented a skit that showed a young Palestinian pretending to stab two Israeli soldiers.

Conservative Party legislator Eric Pickles, Britain’s special envoy for post-Holocaust matters, told the Daily Mail that Corbyn has failed to renounce the “repugnant” group.

Corbyn’s office released a statement Sunday defending his involvement with the group, which he said was recognized by the UN Relief Agency and the British Charities Commission.

The statement said Corbyn has supported Interpal’s humanitarian work and in 2013 toured Gaza on an “Interpal backed” humanitarian trip that included a Conservative and a Liberal Democrat legislator.

The issue of anti-Semitism within Labour flared up in the last week when Labour legislator Naz Shah was suspended for posting anti-Israel material before she was elected to Parliament. That prompted former London Mayor Ken Livingstone to defend her by saying that Nazi leader Adolf Hitler had been a Zionist early in his political career.

Livingstone was quickly suspended from his role on the party’s executive council, but his provocative comments led Corbyn to set up an independent review of anti-Semitism and other racism within the party, which was soundly defeated in last year’s general election by Prime Minister David Cameron’s Conservatives.

The row has snowballed into a full grown crisis, with grumblings beginning to be heard of a possible party mutiny over Corbyn’s leadership.

The Times newspaper reported late Sunday that some top Labour politicians were discussing whether to resign in the coming weeks in protest over Corbyn’s handling of the anti-Semitism crisis.

But Labour legislator Diane Abbott, a close ally of Corbyn, said Sunday the party is being unfairly attacked by its political enemies.

“It is a smear to say that the Labour Party has a problem with anti-Semitism. It is not fair on ordinary Labour Party members,” Abbott told the BBC’s Marr. “Two hundred thousand people have joined the Labour Party. Are you saying that because there have been 12 reported incidents of hate speech online, that the Labour Party is somehow intrinsically anti-Semitic?”

Abbott, who helps set the party’s international development policies, made her comments as debate on whether the frequent criticism of Israeli government policies from Labour members had crossed over into anti-Semitism overshadowed last-minute electioneering for a mayoral race for London slated for Thursday.

Labour Party mayoral candidate Sadiq Khan, who is leading in pre-election polls against Conservative Jewish candidate Zac Goldsmith, said Livingstone’s comments had made his path to victory tougher.

“I accept that the comments that Ken Livingstone has made make it more difficult for Londoners of Jewish faith to feel that the Labour party is a place for them,” he told The Observer newspaper.