WASHINGTON -- The Pentagon on Tuesday said that the U.S. has spotted activity in Syria, at a specific aircraft shelter at Shayrat airfield, which is known to be associated with chemical weapons, CBS News' David Martin reports.

Pentagon spokesman Capt. Jeff Davis said U.S. intelligence had detected an increased level of activity at Shayrat over the last couple days, including increased aircraft activity. He said the evidence of preparations for an attack "became more compelling" within last 24 hours and that it was "strongly suggestive of intent" to conduct another chemical attack. Shayrat airfield is known to be associated with chemical weapons.

A State Department official confirmed the Pentagon's guidance about the increased level of activity at Shayrat over the last couple of days, according to CBS News' Margaret Brennan. However, the State Department is stopping short of attributing intent or of mirroring the Pentagon's language.

The Pentagon's statement gave weight to an ominous warning from the White House Monday night, in which press secretary Sean Spicer said the U.S. "has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children." The statement also threatened Syrian President Bashar al-Assad, saying that if he "conducts another mass murder attack using chemcial weapons, he and his military will pay a heavy price."

Spicer said the activities were similar to preparations taken before the April attack, but he provided no evidence or further explanation. The Monday evening statement caught many administration officials by surprise, although Secretary of State Rex Tillerson was aware of the statement before it was sent.

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U.N. Ambassador Nikki Haley said that the White House also aimed to send a message to other leaders, in addition to Assad.

I believe that the goal is at this point, was not just to send Assad a message," she told lawmakers in a hearing on the U.N. budget, "but to send Russia and Iran a message that if this happens again, we are putting you on notice. And my hope is that the president's warning will certainly get Russia and Iran to take a second look, and I hope that it will caution Assad from the fact that we don't want to see innocent men, women and children hurt again."

Currently, the Navy has four ships armed with cruise missiles in the eastern Mediterranean, according to Martin. The cruise missile strike in April was carried out by two ships. A U.S. official says no strike order has been issued, but the ships are planning, in case they get an order to act.

The U.S. accusation and ominous warning marked a further escalation of tensions in a country where the U.S. is using Syria Arab and Kurdish proxy soldiers to combat the Islamic State group in its remaining strongholds, even as Russia and Iran work to prop up President Bashar Assad, who has gained the upper hand in a long civil war.

Assad's government and Russia dismissed the Trump administration's allegation that Damascus was preparing a new chemical weapons attack. Russian President Vladimir Putin's spokesman Dmitry Peskov said that "such threats to Syria's legitimate leaders are unacceptable." Russia is Assad's key backer and sided with him when he denied responsibility for a chemical weapons attack that killed dozens of people in Idlib province on April 4.

The U.S. responded to that attack by hitting the airfield with dozens of cruise missiles. In the days following, Defense Secretary Jim Mattis cautioned that the U.S. was prepared to take further action if Syria repeated such chemical weapons use.

Several State Department officials typically involved in coordinating such announcements said they were caught completely off guard by the warning, and it appeared the underlying intelligence information was known only to a small group of senior officials. Typically, the State Department, the Pentagon and U.S. intelligence agencies would all be consulted before the White House issued a declaration sure to ricochet across foreign capitals.

The officials weren't authorized to discuss national security planning publicly and requested anonymity.

On Tuesday, the deputy White House spokeswoman, Sarah Huckabee Sanders, said "all relevant agencies," including the Pentagon, State Department and key intelligence agencies, "were involved in the process from the beginning. Anonymous leaks to the contrary are false."

A non-governmental source with close ties to the White House said the administration had received intelligence that the Syrians were mixing precursor chemicals for a possible sarin gas attack in either the east or south of the country, where government troops and allied forces have faced recent setbacks.

A senior Russian lawmaker dismissed the U.S. warning as "provocation."

Frants Klintsevich, first deputy chairman of the defense and security committee in the upper chamber of the Russian parliament, accused the United States of "preparing a new attack on the positions of Syrian forces."

The U.S. strike in April was the first direct American assault on the Syrian government and Mr. Trump's most dramatic military order since becoming president.

Mr. Trump said at the time that the chemical attack crossed "many, many lines," and called on "all civilized nations" to join the U.S. in seeking an end to the carnage in Syria.

Syria denied using chemical weapons. Russia's Defense Ministry said the toxic agents were released when a Syrian airstrike hit a rebel chemical weapons arsenal and munitions factory.

The U.S. attack on a Syrian air base came after years of heated debate and deliberation in Washington over intervention in the bloody civil war. Chemical weapons have killed hundreds of people since the start of the conflict.

CBS News' David Martin and Margaret Brennan contributed to this report.