The top US diplomat for European affairs, Wess Mitchell, has resigned, the State Department confirmed on Tuesday.

In the letter to US Secretary of State Mike Pompeo dated January 4, Mitchell said he had completed his goals for the post and wanted to spend more time with his young family.

Mitchell served as the assistant secretary of state for Europe and Eurasian affairs. The State Department said he would leave the post in mid-February.

Mitchell told The Washington Post, which first reported his departure, that he was leaving for family reasons - mainly to spend more time with his two young children - and not over disagreements with administration policies.

His resignation comes at a time of tensions with some key European allies over the Trump administration's decisions to withdraw from the Iran deal, the global climate pact and, most recently, to withdraw troops from Syria.

His decision to leave was also announced just over a month after Pompeo delivered a speech in Brussels, the home of the European Union, questioning the value of multilateral institutions like the EU and international organisations favoured by Europe, including the IMF and World Bank.

'Outstanding job'

Pompeo said on Twitter that Mitchell had done "an outstanding job as Assistant Secretary".

"I have valued his counsel and wisdom as he has led our European team in this administration. I wish him and his wife Elizabeth, who is also a committed public servant, much happiness with their two young children," Pompeo tweeted.

In his resignation letter, Mitchell said he had helped to craft the administration's European strategy and reorganise staff in the European bureau of the State Department.

"I am proud of what we have accomplished in creating and beginning to implement the Europe Integrated Strategy in support of the National Security Strategy and National Defense Strategy," he wrote. "The emphasis that these strategies place on the need to prepare our country and the nations of the West for sustained competition with big-power rivals is both urgently necessary and long-overdue," he added.

Mitchell took up the job in October 2017 under former Secretary of State Rex Tillerson after 12 years at a think-tank focused on Central European issues.