UPDATE 4-Trump defends Mexico migration deal and pledges more detail

(Adds color from Mexico's southern border)

By Lesley Wroughton and Doina Chiacu

WASHINGTON, June 9 (Reuters) - U.S. President Donald Trump defended his administration's deal with Mexico against criticism that there were no major new commitments to stem a flow of Central American migrants crossing into the United States, and said on Sunday more details would soon be released.

Key aspects of the agreement are still unclear, including whether Mexico has pledged to buy more U.S. agricultural products and if the deal materially expanded a previous commitment by Mexico to more vigorously police its southern border with Guatemala.

Top Democratic senators said many aspects of the deal were not new, while the Mexican ambassador to the United States declined to confirm whether it contained any commitments on agricultural goods.

The deal, announced on Friday after three days of talks in Washington, averted Trump's threatened imposition of 5% import tariffs on all Mexican goods that had been due to start on Monday unless Mexico committed to do more to help reduce an increase in migrants arriving at the U.S. southern border.

"Mexico was not being cooperative on the Border...and now I have full confidence, especially after speaking to their President yesterday, that they will be very cooperative and want to get the job properly done," Trump said on Twitter.

"Importantly, some things not mentioned in yesterday press release, one in particular, were agreed upon. That will be announced at the appropriate time," he added.

Trump did not elaborate and the White House and State Department did not immediately respond to requests for comment.

Friday's joint communique broadly sketched the terms of the agreement, but contained few details.

The Mexican peso jumped against the dollar in Asian markets and U.S. equity futures rose on Sunday following the deal.

The agreement expedites a program known as the Migration Protection Protocols that was announced in December. That program sends migrants seeking asylum in the United States to wait in Mexico while their cases are being processed.

Friday's agreement would see the United States immediately expand the implementation of the MPP across its entire 2,000-mile (3,220-km) southern border, the State Department said on Friday.

But as of Sunday night, U.S.-bound asylum seekers were not yet being sent back to additional Mexican border towns, three Mexican officials said.

Officials in the Mexican border states of Sonora, south of the U.S. states of Arizona and New Mexico, and Tamaulipas, south of Texas, said they were unaware of plans to expand the number of border crossings where the asylum seekers must be returned to Mexican territory to await processing.