President Donald Trump weighed in on the Syrian government's allegedly conducting a new chemical attack against its own citizens. The alleged attack on Saturday killed 40 civilians, according to reports to The Associated Press that the news service has not independently confirmed.

The president in his tweets Sunday morning blasted Russian President Vladimir Putin and the government of Iran for backing the regime of Syrian President Bashar al-Assad in his civil war with rebel forces. Trump also criticized former President Obama, suggesting that he could have taken action to end the Syrian civil war.

A Republican senator responded to Trump's tweets by saying he needs to follow through on his threats to respond to the alleged use of chemical weapons.

“If he doesn't follow through and live up to that tweet, he's going to look weak in the eyes of Russia and Iran so this is a defining moment,” Republican Sen. Lindsey Graham of South Carolina told ABC News Chief Global Affairs Correspondent and co-anchor Martha Raddatz on "This Week” Sunday.

“Mr. President, you need to follow through with that tweet, show a resolve that [former president Barack} Obama never did to get this right,” Graham said on "This Week."

An adviser to Trump who also appeared on "This Week" said he would take nothing "off the table" in predicting the U.S. response to the alleged chemical attack.

“I wouldn’t take anything off the table,” White House homeland security and counterterrorism adviser Tom Bossert told Raddatz. “These are horrible photos [from the alleged attack]. We’re looking into the attack at this point.”

The Trump adviser said the use of chemical weapons is "one of those issues in which every nation, all peoples have all agreed, and have agreed since World War II, that this is an unacceptable practice."

“The president’s senior national security cabinet have been talking with him and with each other all throughout the evening and this morning and myself included" about the alleged chemical attack, Bossert said.

ABC News contributor Steve Ganyard said that if Trump does strike Syria in response, it could take days to happen. The U.S. strike against a Syrian air base in 2017 came on April 7, three days after a chemical attack the killed approximately 100 civilians, according to a State Department estimate.

"That strike was very limited and targeted aircraft in hardened bunkers in remote areas," Ganyard said. "It was a warning more than punishment."

A U.S. strike against Syria could potentially send "a message to Damascus that would carry, loud and clear, all the way to Pyongyang," Ganyard said. Trump and North Korean leader Kim Jong Un plan to meet for negotiations over North Korea's nuclear program.