(CNN) A group of bipartisan senators on Wednesday introduced legislation aimed at punishing Beijing for blocking American citizens from leaving that country, on the same day that the White House signed an initial trade deal with China,

The bill introduced Wednesday takes aim at China's use of "exit bans" and comes as Chinese authorities have prevented an American family from leaving more than a year and a half through the use of such a ban. That case has risen to the highest levels of the US and Chinese governments, according to a source familiar.

The Liu siblings -- Cynthia, 28, and Victor, 20 -- and their mother, Sandra Han, traveled to China in June 2018 to visit their ailing grandfather and have been held there since.

The brother and sister have made public pleas about their plight, and in a letter to Sen. Edward Markey last week, Cynthia Liu implored the Massachusetts Democrat for his help.

"We have now become political pawns, and only with the forceful advocacy and involvement of the United States government will this nightmare be able to end," she wrote in a letter obtained by CNN. "This is why we are writing to you, because Victor and I cannot take any more of this."

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