U.S. energy companies are planning more layoffs, asset sales and financial maneuvers to deal with a recent, sudden drop in U.S. crude-oil prices to under $50 a barrel, the lowest level in four months.

The companies had been banking on a rebound in oil prices in the second half of 2015 after falling sharply late last year. Prices began to regain ground in the spring, rising so quickly that some American producers started hiring back drilling rigs to pump more crude. That speedy return to the oil patch and the threat of new...