U.S. President Donald Trump on Wednesday signed into law a sanctions bill on Russia, Iran and the Democratic People's Republic of Korea (DPRK), but he criticized the bill as "seriously flawed."

In a statement released by the White House, Trump said while he favoured tough measures to "punish and deter bad behavior" by Iran and the DPRK and supported making clear to Russia that "America will not to tolerate interference in our democratic process," he criticized the legislation for including "unconstitutional provisions" and being "seriously flawed."

"Still, the bill remains seriously flawed -- particularly because it encroached on the executive branch's authority to negotiate," said Trump.

Unlike previous sanctions bills, the new legislation grants U.S. lawmakers power to block Trump from unilaterally lifting sanctions on Russia.

The bill was approved overwhelmingly early this month by the U.S. Congress despite the Trump administration calling on lawmakers to grant the White House "flexibility" in dealing with Russia.

"By limiting the Executive's flexibility, this bill makes it harder for the United States to strike good deals for the American people," said Trump in the statement.

Despite his objections, Trump said he signed the bill "for the sake of national unity."

"It represents the will of the American people to see Russia take steps to improve relations with the United States," said Trump.

In retaliation for the new sanctions legislation, Moscow had already ordered the U.S. diplomatic mission in Russia to cut its staff by 755 people.

Russian President Vladimir Putin on Sunday announced Moscow's decision to reduce the U.S. diplomatic staff in Russia by 755 people, including U.S. and Russian nationals, by Sept. 1.