A summer airfare war is heating up as one of the worst downturns in travel is leaving airlines scrambling to fill seats.

Fares for summer trips are often among the highest of the year and start rising in the spring, but not this year. With business travel plummeting, airlines are pulling back this year and offering some of the lowest plane tickets in recent memory.

“This is a whopper of an airfare sale,” said Tom Parsons, chief executive of air travel Web site Bestfares.com. “They are doing everything they can to make you fly.”

Southwest Airlines on Thursday launched the first volley, announcing it was slashing fares across its network for travel up to Aug. 14.

The fares – as low as $49 one-way from Los Angeles to San Francisco for 14-day advance purchase – apply for travel Monday through Thursday and Saturday, making the sale one of the more extensive so far. Reservations must be made by April 6.

Major airlines including American, United and Delta matched the fares on most of the routes flown by Southwest.

The latest fare deals are the broadest so far and include travel days that airlines often try to exempt from the discounts.

The offers include deals for travel any day of the week except Fridays and Sundays, giving travelers five days out of the week to qualify for a low fare. They also extend through much of the summer. In the past, most fare deals stopped on or before June.

With taxes and fees, a round-trip flight from Los Angeles to Baltimore-Washington International Airport in late June was being offered at $218 Thursday by Southwest. United and US Airways quickly beat it by offering a fare of $211.

Some typical caveats remained for peak holiday travel days: There were few deals for travel during the Memorial Day and Fourth of July holidays. Also, fares on certain routes with limited service remained relatively high.

Airlines that have international flights are also slashing fares for trips to other countries with round trip plane tickets from Los Angeles to London falling to below $600, compared to about $1,000 last summer. United Airlines on Thursday was offering a round-trip flight to Moscow from Los Angeles for $598 on certain days of travel.