The United Nation's special envoy to Syria said Wednesday evidence shows some kind of chemical “substance” was used in an attack last week that killed hundreds of people, according to reports.


But the envoy, Lakhdar Brahimi, emphasized that any military strike against the war-torn nation must be approved by the U.N. Security Council.

A U.N. inspection team is investigating an alleged poison gas attack by Syrian government forces in the suburbs of Damascus last week.

“It does seem like some kind of substance was used,” Brahimi told reporters in Geneva, but “international law says that any U.S.-led military action must be taken after” agreement in the Security Council.

As horrific images of the alleged attack came to light this week, pressure has been mounting for a military strike against Syrian President Bashar Assad.

The White House said Tuesday that President Obama is not seeking “regime change” in Syria, instead weighing a reaction to the violation of “an international standard” barring the use of chemical weapons.

David Axelrod, a former Obama campaign adviser, said the president is weighing his next move “very, very carefully.”

“This isn’t about regime change; this is about responding to a specific heinous act,” Axelrod said Wednesday on MSNBC’s “Morning Joe.”

White House press secretary Jay Carney on Tuesday said that there was “no military solution to the conflict in Syria,” suggesting a U.S. strike would be limited. But he added that “there must be a response” to the latest attack.

Although Carney insisted that no decision had yet been made, Secretary of Defense Chuck HagelCharles (Chuck) Timothy HagelInterpreter who helped rescue Biden in 2008 escapes Afghanistan Overnight Defense & National Security — Pentagon chiefs to Congress: Don't default Pentagon chiefs say debt default could risk national security MORE told the BBC on Tuesday that the U.S. military was “ready to go” if orders came from the president to begin the strikes.

Meanwhile, nearly two dozen House members have signed onto a letter demanding President Obama seek authorization from Congress before launching any such strike.

“Engaging our military in Syria when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers that is clearly delineated in the Constitution,” wrote Rep. Scott RigellScott RigellSpanberger's GOP challenger raises over .8 million in third quarter Ex-Rep. Scott Taylor to seek old Virginia seat GOP rushes to embrace Trump MORE (R-Va.).