Frustrated by an “appalling counterproposal” earlier this week, the head of the union representing 340,000 UPS workers said a strike is imminent and gave the shipping giant a Friday deadline to improve its offer.

The International Brotherhood of Teamsters walked away from negotiations Wednesday, demanding that UPS give its “last, best, and final offer” no later than June 30.

Teamsters officials did not say what time the Friday deadline was or what actions it might take if it is not met.

“The largest single-employer strike in American history now appears inevitable,” said Teamsters General President Sean O’Brien, who accused UPS executives of hoarding profits instead of sharing them with workers.

“Executives at UPS, some of whom get tens of millions of dollars a year, do not care about the hundreds of thousands of American workers who make this company run,” O’Brien said.

In a brief statement, United Parcel Service said it has offered significant changes to its initial financial proposal and that “Reaching consensus requires time and serious, detailed discussion, but it also requires give-and-take from both sides.”

Negotiations on the national contract began in April. The current contract expires July 31.

Earlier this month, the Teamsters said 97% of unionized workers voted for a strike authorization, which the union urged to give it more leverage during negotiations with the company.

The Teamsters represent more than half of the company’s workforce in the largest private-sector contract in North America. If a strike occurs, it would be the first since a 15-day walkout by 185,000 workers crippled the company a quarter century ago.

UPS has pushed back on those claims by boasting that is provides workers with industry-leading pay and benefits.

Unionized UPS workers are still upset about the current contract, which they feel was forced on them by prior union leadership in 2018 based on a technicality. The contract created two hierarchies of workers with different pay scales, hours and benefits. The union wants it eliminated.

Two weeks ago, the union and the company announced they reached a tentative agreement to equip more trucks with air conditioning equipment, a major sticking point. UPS said it would add air conditioning to U.S. small delivery vehicles purchased after January 1, 2024.

UPS delivers around 25 million packages a day, representing about a quarter of all U.S. parcel volume, according to the global shipping and logistics firm Pitney Bowes. That’s about 10 million parcels more than it delivered each day in the years leading up to the pandemic.

UPS profits have soared since the pandemic began in 2020 as millions of Americans grew to rely on delivery to their doorstops.

Annual profits at UPS in the past two years are close to three times what they were pre-pandemic. The Atlanta company returned about $8.6 billion to shareholders in the form of dividends and stock buybacks in 2022, and forecasts another $8.4 billion for shareholders this year.