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The US ambassador to Mexico has announced that she is stepping down in May.

Roberta Jacobson made the announcement in a tweet in Spanish on Thursday, “I will be departing at the beginning of May, in search of other opportunities.”

The resignation comes at a time when US relations with Mexico have been strained over issues related to trade and migration and, Jacobson ended her tweet stating, “We are Stronger Together!”

“I do it knowing the US-Mexico relationship is strong and vital and that this amazing Mission Mexico team will continue to ensure it remains so,” she adds. “At this time, I have no news to share on my successor.”

A source close to Jacobson said, “it was a tough job and she is looking for new opportunities outside government after 31 years.”

Jacobson is an experienced US diplomat who was nominated to the position in 2015 by then-President Barack Obama and confirmed the following spring.

She was closely involved with the Obama administrations’ efforts to reopen diplomatic relations with Cuba in her previous role at the helm of the agency’s western hemisphere bureau as well as efforts to broker a peace deal between the Colombia government and the guerrilla group FARC.

Last weekend, Mexican President Enrique Peña Nieto called off an official trip to Washington to meet with President Donald Trump after a tense phone call.

CNN previously reported Peña Nieto was tentatively planning the trip for March, a White House official said, but the official confirmed that the trip was put on hold following the phone call, which took place on February 20.

State Department Spokeswoman Heather Nauert told reporters the agency is grateful for Jacobson’s service.

“She has certainly done a tremendous job in representing US interest with the government of Mexico,” said Nauert.

Jacobson is the latest in a series of high-ranking US diplomats to step down in recent months. Earlier this week, Ambassador Joseph Yun, the top diplomat in charge of North Korea policy, announced his resignation. In January, the US ambassador to Panama, John Feeley, also announced he would be stepping down.

In November, the head of the American Foreign Service Association, a union representing US diplomats, said the State Department’s senior tiers are being “depleted at a dizzying speed.”

The top Democrat on the Senate Foreign Relations Committee, Sen. Bob Menendez (D-NJ), issued a statement in which he called Robertson’s departure, “yet another critical moment in the exodus of expertise from a State Department suffering under poor management and dangerous political guidance.”

“If you look at amount of time that many of these individuals have invested in their careers at the State Department … that’s a tremendous amount of time to be working in any industry, building, service, government agency, or department,” Nauert said, refuting the notion the department is experiencing a damaging loss of senior personnel. “People are choosing to retire for personal reasons, and that’s perfectly fine with us.”

Former Secretary of State John Kerry praised Jacobson on Twitter for her contributions to US foreign policy.

“Whether it was Mexico, Colombia, Cuba and more – Roberta demonstrated the difference diplomacy can make to advance American interests and build peace,” he wrote Thursday.

“On a personal level, she also made Alan Gross’s cause her own,” Kerry continued, referencing the case of a US contractor who was released from a Cuban prison in 2014 as part of a deal negotiated between the two governments. “Was lucky to have her on America’s team.”