The US State Department said a vote by Senegal's parliament to delay presidential elections until December "cannot be considered legitimate", urging the country to move forward with the election "in accordance with the Constitution".

Senegal was plunged into its worst political crisis in decades after President Macky Sall on Saturday announced a postponement to the poll originally scheduled for February 25.

Lawmakers voted almost unanimously in favour of the delay on Monday night, but only after security forces stormed the chamber and removed some opposition deputies, who were unable to cast their votes.

"The United States is deeply concerned by actions taken to delay Senegal's February 25 presidential election, which run contrary to Senegal's strong democratic tradition," Matthew Miller, a US State Department spokesman, said in a statement published Tuesday.

"We are particularly alarmed by reports of security forces removing by force parliamentarians who opposed a bill to delay the election, resulting in a National Assembly vote that cannot be considered legitimate given the conditions under which it took place".

He also called on the government to respect freedoms of peaceful assembly and expression, including for members of the press.

"The United States will remain engaged with all parties and regional partners in the days ahead," he added.