NATO (North Atlantic Treaty Organisation) leaders pose for a group photo at the NATO Summit in London, Britain, on Dec. 4, 2019. (Photos: Xinhua)

NATO Secretary General Jens Stoltenberg (L) talks with British Prime Minister Boris Johnson during the official handshake ceremony at NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L) talks with British Prime Minister Boris Johnson during the official handshake ceremony at NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L) talks with British Prime Minister Boris Johnson during the official handshake ceremony at NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L), British Prime Minister Boris Johnson (R) welcome U.S. President Donald Trump at the official arrivals for NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L), British Prime Minister Boris Johnson (C) welcome U.S. President Donald Trump at the official arrivals for NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L), British Prime Minister Boris Johnson (R) welcome U.S. President Donald Trump at the official arrivals for NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

U.S. President Donald Trump arrives for the official handshake ceremony at NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.

NATO Secretary General Jens Stoltenberg (L), British Prime Minister Boris Johnson (R) welcome U.S. President Donald Trump at the official arrivals for NATO (North Atlantic Treaty Organisation) Summit in London, Britain on Dec. 4, 2019.