President Donald Trump claimed Monday that Lockheed Martin had knocked $600 million off the cost of the F-35 Joint Strike Fighter, a government contract he has sharply criticized for cost overruns.

'I appreciate Lockheed Martin for being so responsive,' Trump told reporters at the White House Monday.

Speaking from the Oval Office, Trump added that the company was doing a great job on the program.

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President Donald Trump today claimed that he was able to get $600 million off the price of the F-35 line of fighter jets

'I appreciate Lockheed Martin for being so responsive,' Trump told reporters at the White House Monday

It was a change in tune from tweets he sent out in December, in which he criticized the pricey Lockheed jets and asked Boeing to 'price-out' a competitor.

'Based on the tremendous cost and cost overruns of the Lockheed Martin F-35, I have asked Boeing to price-out a comparable F-18 Super Hornet!' Trump tweeted.

In after-hours trading after Trump had written the tweet, Lockheed's shares fell 2 percent and Boeing's rose 0.7 percent.

However, the competitor the president-elect called for, the F-18 Super Hornet, lacks the stealth capabilities of the Lockheed planes, and are a significantly older model.

'They're two completely different aircraft from different generations,' Phillip Carter, a senior fellow at the Center for New American Security, told the Guardian. 'It's like comparing an old jeep to a Humvee.'

The Lockheed planes were costing about $100 million each to build, with the total cost running at $400 billion.

Earlier this month, however, CEO Marillyn Hewson said the company was 'very close to a deal' to lower the cost of the program.

On Friday, Trump's new Defense Secretary James Mattis ordered a review of the program to 'determine opportunities to significantly reduce the cost.'

Lockheed tweeted that the company 'welcomes the appropriate focus on affordability and capability.'

'We are confident such a thorough and objective analysis will show that only the F-35, with its advanced stealth and sensors, can meet the 21st century air superiority requirements of all of our military services,' the statement read.