The government is encouraging unions to withdraw safe staffing levels during strikes, GMB has claimed.
GMB, which has over 15,000 ambulance workers as members, said they feel "ignored" by ministers after the Royal College of Nursing was invited for new pay talks but other health unions were not.
National Secretary Andrew Prendergast suggested the RCN had succeeded in its drive for fresh talks because it had threatened to scrap an agreement known as a derogation which would ensure there were enough nurses to deal with emergencies.
"We've tried to strike responsibly," he said, "we've had full derogations, the RCN has threatened to remove the derogations and they've been pulled in for talks - and what message does that send to our members?"
GMB national secretary on how strikes could be escalated:
Mr Prendergast added: "Our members are now asking quite legitimately, 'why are we continuing to do this?'
"We've always been in the position that this is a dispute about pay, we're not wanting to put the public at risk, but unfortunately the actions of the government seem to be about exacerbating this dispute, rather than resolving it."
The union leader told ITV News Westminster Producer Jack Abbey that GMB members were "already saying they want to ramp up the dispute" and the recent news of RCN talks had made their mood go from bad to "a lot worse".
The RCN agreed to call off its strikes planned from March 1 to 3 after being called for talks, but the other unions announced more.
Trade union Unison announced another strike of ambulance and other health workers on Wednesday, with a walkout planned for March 8, while further GMB ambulance strikes are planned for March 6 and 20.
The Royal College of Midwives said the government's "divide-and-rule tactic...will not work", with Executive Director Dr Suzanne Tyler saying ministers "need to focus on all NHS staff and negotiate with all of their unions".
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Mr Prendergast added that the move by government to negotiate with the RCN but not all NHS unions "threatens to, frankly, pour petrol over the bonfire".
The UK's other health unions are angry they have not been called in for talks along with the RCN because the NHS has one pay structure from the Department for Health and Social Care which is not divided into different unions.
Mr Prendergast told ITV News it "displays a complete lack of understanding about how how industrial relations work in the NHS - you can't just do a deal with one union".
Downing Street declined to comment on the fresh talks between the Royal College of Nursing and the Government, stressing it was right that talks took place “privately”.
No 10 said that it wanted a deal that is “fair” for all taxpayers.
The prime minister’s official spokesman said: “We are now in a period of discussion that start today and will continue in the coming days, so I won’t be commenting on the detail of those discussions while they are taking place.
“We want to secure a deal that is fair for all taxpayers. That is our ambition. In the first instance, we haven’t had the first meeting yet, so we will let that happen.”
No 10 declined to get into any details of the talks.
“I think given there are discussions, we are not going to get into what is on the table. We think it is right to allow these talks to take place privately, as I believe does the RCN.”