DALLAS, United States (AP) — Pilots at American Airlines have voted to authorise a strike, and Southwest Airlines pilots are preparing to join them, as unions put more pressure on the airlines to approve new contracts with hefty pay raises.

The actions by pilots are highly unlikely to lead to walkouts anytime soon.

Federal law prohibits airline unions from striking without the approval of a US mediation board – a rare step that has not occurred with negotiations at either American or Southwest. Congress and the president can also act to prevent a strike if one appears imminent.

Airline unions like to take strike votes, however, which they believe increase their leverage at the bargaining table. American, Southwest and United Airlines are under pressure to match or beat terms that rival Delta Air Lines accepted with its pilots, who earlier this year won 34 per cent raises over a four-year contract that will cost Delta about $7 billion.

Some smaller airlines have experienced pilot shortages, and the largest carriers have all strained to hire and train enough new pilots to meet surging travel demand. That gives the unions more bargaining power than usual.

At American Airlines, the Allied Pilots Association said Monday that more than 96 per cent of its 15,000 members voted, and among those who did, 99 per cent favoured authorising the union to call for a strike.

A spokeswoman for the Fort Worth, Texas-based airline said American is confident that it can reach an agreement with the union quickly.

“We understand that a strike-authorisation vote is one of the important ways pilots express their desire to get a deal done, and we respect the message of voting results,” said the spokeswoman, Sarah Jantz.

The American pilots’ union said it is also seeking scheduling changes that union officials say will improve efficiency and prevent the kind of widespread delays and cancellations seen last summer.

“The summer travel season is almost here, and we’re all wondering whether this will be another summer of uncertainty for American Airlines,” union President Ed Sicher said.

American pilots planned to picket later Monday at airports around the country.

Members of the Southwest Airlines Pilots Association began voting Monday on a strike-authorisation measure that is expected to pass easily.

Adam Carlisle, Southwest’s president of labour relations, said the vote will not affect the Dallas-based airline’s operations. He said negotiations are scheduled to resume this week with help from federal mediators, and the airline hopes to reach an agreement with pilots that “places them competitively in the industry”.