LONDON, Dec 3 - One of the candidates to become the U.N.'s first female secretary-general, senior U.N. official Rebeca Grynspan, has pledged to restore trust in the organisation as it faces heavy criticism from U.S. President Donald Trump.

The process to elect the next U.N. chief formally kicked off last week as member states were asked to nominate candidates to take over the role from Antonio Guterres of Portugal from January 1, 2027.

Grynspan, secretary-general of the U.N. Conference on Trade and Development and a former vice president of Costa Rica, is among the publicly declared candidates for U.N. leadership. If elected, she would assume the role during Trump's tenure as U.S. president.

Trump has maintained the same wary stance on multilateralism that was a hallmark of his 2017-2021 first term and has accused the U.N. of failing to help him broker peace in various conflicts. He describes the U.N. as having "great potential", but says it has to get its "act together".

U.N. leadership has pushed back against Trump's remarks, saying it works to implement reforms and that U.S. support for its institutions has saved lives.

Grynspan, 69, told Reuters in an interview in London on Tuesday that Trump's critiques could be "constructive" and not "destructive," emphasising that the U.N. should strive for improvement: "The U.N. has to embrace reform, not be defensive about reform."

Asked whether Trump's criticism of the U.N. had merit, she said it was important to acknowledge a "weakening of trust" in the organisation and to work toward restoring confidence in an institution she called crucial for global stability.

The U.N. has helped create a world with less poverty and lower maternal and infant mortality rates, among other achievements, she added.

"Institutions have to be able to look at themselves and... admit that they don't do everything right," she said. "What we don't want is to throw away all the experience and infrastructure the U.N. has built (over) these 80 years that has served the world pretty well."