0350 GMT January 18, 2018
"The United States of America stands behind Japan, its great ally, 100 percent," Trump said in brief remarks during a joint press conference with Abe in Florida late on Saturday.
The Japanese leader denounced the new launch as "intolerable."
"North Korea's most recent missile launch is absolutely intolerable. North Korea must fully comply with the relevant UN Security Council resolutions," Abe said through a translator.
The remarks came a day after the two leaders held a joint press conference at the White House before heading to Trump's private mansion in Palm Beach for the weekend.
US Strategic Command systems tracked a test launch over North Korea and into the Sea of Japan, and determined it was a medium- or intermediate-range ballistic missile, the Pentagon said on Saturday.
The missile test is the first conducted by North Korea since Trump took office on January 20.
An official with the South Korean Ministry of Defense also confirmed the test to CNN but said it was a ballistic missile.
The test of an intercontinental-range system would be especially provocative because the White House views it as an attempt by the North to develop the capability to strike the United States.
Trump has pledged to get tough on North Korea that tested several nuclear devices and ballistic missiles last year.
"Our assessment is that it is part of a show of force in response to the new US administration's hardline position against the North," the office of South Korea’s Joint Chiefs of Staff said in a statement, according to Reuters.
A White House spokesman said Trump was briefed on the new launch. However, the US president declined to comment when asked by reporters about the missile test as he posed for pictures with the Japanese leader ahead of a dinner in Florida.
Trump told Abe on Friday that defending against Pyongyang’s “nuclear and missile threat” was a “very, very high priority.”
Trump's national security adviser, Michael Flynn, spoke to his South Korean counterpart Kim Kwan Jin after the latest launch. They both condemned the test and agreed "to seek all possible options" to deter North Korea.
The US maintains 47,000 military service members in Japan, mostly on the southern island of Okinawa.
Throughout his presidential campaign, Trump frequently criticized Washington’s commitment to its defense agreement with Japan.