New US Secretary of Defense James Mattis reassured his British counterpart on Monday that Washington has an "unshakeable commitment" to NATO, despite President Donald Trump previously casting the military alliance as obsolete.

During a phone call with Michael Fallon on his first full day in office, Mattis "emphasized the United States' unshakeable commitment to NATO," Pentagon spokesman Captain Jeff Davis said in a statement.

Ahead of his inauguration, Trump told two European newspapers he had long warned that NATO had "problems."

"Number one, it was obsolete, because it was designed many, many years ago," he said, referring to its Cold War, post-World War II origins.

"Number two, the countries aren't paying what they're supposed to pay."

Trump said in his inaugural address Friday that the United States had "subsidized the armies of other countries," highlighting a common cause of friction in the 28-nation alliance.

In response to Trump's remarks, German Chancellor Angela Merkel warned that Europe has to take responsibility for itself.

During his phone call with Fallon, Mattis also "emphasized the United States and the United Kingdom will always enjoy a uniquely close relationship, reflected in our defense ties which are a bedrock of US security," according to his spokesman.

British Prime Minister Theresa May will visit Washington on Friday. She will be the first foreign leader to meet with Trump since he took office last week.

Trump committed to Egypt military assistance
Washington (AFP) Jan 23, 2017 - US President Donald Trump is committed to providing military assistance to Egypt, White House spokesman Sean Spicer said Monday, after the new Republican leader spoke with counterpart Abdel Fattah al-Sisi.

Trump "underscored the United States remains committed to the bilateral relationship, which helped both countries overcome challenges in the region for decades," Spicer said.

Washington's annual $1.3 billion in military aid was briefly suspended under Trump's predecessor Barack Obama in 2013, following the ouster of then president Mohamed Morsi, but was fully reinstated in 2015.

Ties have nonetheless been fraught amid disagreements over a rights crackdown and Egypt's support for renegade Libyan general Khalifa Haftar.

Spicer said "the two leaders discussed a visit to the United States in the future," and stressed counterterror cooperation.

"The president committed to continuing military assistance to Egypt and working with Egypt to ensure that assistance most effectively supports the Egyptian military's fight against terrorism," Spicer said.