After meeting with China's president Xi Jinping at the G-20 summit, President Donald Trump announced that he and his Chinese counterpart had among other issues, come to an agreement help stem the flow of illegal fentanyl from China to the United States.

The White House said in a statement Saturday that China has agreed to designate fentanyl as a controlled substance, "meaning that people selling Fentanyl to the United States will be subject to China's maximum penalty under the law."

The White House praised the move as "a wonderful humanitarian gesture" by President Xi and the news has been greeted with cautious optimism by lawmakers and experts.

But terms of the so-called agreement are as yet unclear. Chinese state-controlled media outlets on Sunday did not explicitly say that fentanyl would be designated as a controlled substance as per the White House statement, reporting instead that China would only move toward controlling fentanyl.

"My meeting in Argentina with President Xi of China was an extraordinary one," President Trump tweeted on Monday as observers tried to parse how the two giants would actually come to terms on trade and other issues discussed in Argentina. The Trump administration has put a pause on escalating tariffs as the two sides negotiate for the next 90 days.

The fentanyl news comes at a time when lawmakers are scrambling to control the country's growing opioid epidemic.

In a report released last week, the Center for Disease Control and Prevention (CDC) said that overdose deaths involving synthetic opioids such as fentanyl increased by 45 percent between 2016 and 2017. Fentanyl, according to the CDC, is up to 100 times more potent than morphine and many times that of heroin.

In October, President Trump signed the SUPPORT for Patients and Communities Act into law, which aims to expand opioid addiction treatment and fund research on non-opioid painkillers. The law also incorporates the STOP Act, which requires the U.S. Postal Service to send advanced electronic data on international packages to Customs and Border Protection, in an aim to stop fentanyl from entering the country via mail.

Members of Congress from both sides of the aisle applauded the move by President Trump but seemed wary that the the Chinese government would follow through.

Sen. Lamar Alexander, R-Tenn., said in a statement that the "president deserves great credit for persuading China to make the selling of fentanyl subject to the maximum penalty under Chinese law." Tennessee has a higher rate of opioid-related deaths than the national average.

"We must do all we can to hold China accountable and ensure that they follow through on this commitment," said Sen. Maggie Hassan, D-N.H., who also co-sponsored the STOP Act.

Fentanyl is the leading cause of overdose deaths in New Hampshire, said Sen. Jeanne Shaheen, D-N.H., in a statement. She called Trump's announcement "an important breakthrough that will help save lives but this commitment must be followed by diligent implementation by the Chinese government."

Sean O'Connor, policy analyst at the U.S.-China Economic and Security Review Commission and author of a recent report on fentanyl trafficking from China, was skeptical that the move would have a large impact.

"After a new substance is controlled in China, manufacturers modify the chemical structure to create 'new' chemical analogues that aren't controlled in China," O'Connor said in a tweet. "It's essentially a game of chemical whack-a-mole where regulators are always one step behind the manufacturers."

He continued, pointing out that "China has made sweeping agreements on fentanyl in past meetings with the U.S. and failed to follow through."