NEW YORK, Feb. 3 -- U.S. oil workers have been staging a large-scale strike rarely seen in decades at nine refineries since Monday, as union leaders failed to agree on a new national contract with major oil companies after less than two weeks of negotiations.

The strike is the first held in support of a national pact since 1980 and impacts plants with a combined 10 percent of the U.S. refining capacity.

The United Steelworkers Union (USW) asked about 3,800 workers at refineries in California, Kentucky, Texas and Washington states to strike shortly after their previous contract expired Sunday midnight.

The USW took the action after rejecting a fourth contract offer proposed by Shell, which leads the negotiations on behalf of refiners. The union said Shell refused to provide a counter offer and the company's representatives had left the bargaining table.

The USW said in a statement that it "had no choice" but to call for a work stoppage and listed a series of problems, including "onerous overtime," "unsafe staffing levels" and the erosion of oil workers' workplace due to outsourcing of work to contractors.

"The industry is the richest in the world and can afford to make the changes we offered in bargaining," USW International Vice President of Administration Tom Conway said in a statement.

The "oil companies are too greedy to make a positive change in the workplace," said Conway, adding that "they continue to value production and profit over health and safety, workers and the community."

Royal Dutch Shell, the lead industry negotiator, said it hoped to "resume negotiations as early as possible," according to reports.

The USW represents about 30,000 workers at 65 U.S. refineries that account for nearly two-thirds of the total U.S. oil output.

Since Jan. 21, the USW has been renegotiating a three-year national contract with Shell in an attempt to double the size of the annual pay increases from the previous agreement, increase healthcare coverage and reduce use of contractors.

Any agreement reached between the two sides would then be used as a pattern for negotiations at a local level.

The nationwide walkouts come at a difficult time for oil companies, which have been cutting costs and reining in spending following a collapse in global crude oil prices, while an increase in oil volumes extracted from shale formations is another contributing factor.

The labor tensions have also led to a 7-percent increase in futures prices of gasoline since Friday amid concerns that refined product supplies could be constrained.

Those affected plants are owned by Shell, LyondellBasell, Marathon Petroleum and Tesoro.

Reports said Tesoro had shut down its Martinez and California refinery in light of the strike while keeping up production in other facilities. The other three companies have also implemented strike contingency plans.

The remaining sites not targeted for a strike will operate under contract extensions that renew every 24 hours until one side in the negotiations decides that they have reached an impasse, according to USW spokeswomen Lynne Hancock.

"We haven't had a work stoppage like this since 1980," she added.