On Monday, President Barack Obama said there would be consequences if Assad made the "tragic mistake" of deploying chemical weapons, and Israeli Prime Minister Benjamin Netanyahu said he agreed with the U.S. position.
"We are of the same opinion, that these weapons should not be used and must not reach terror groups," Netanyahu said.
U.S. intelligence has seen signs that Syria is moving materials inside chemical weapons facilities recently, though it is unsure what the movement means. Still, U.S. officials said the White House and its allies are weighing military options should they decide to secure Syria's chemical and biological weapons.
In July, Foreign Ministry spokesman Jihad Makdissi told a news conference that Syria would only use chemical or biological weapons in case of foreign attack, not against its own people. The ministry then tried to blur the issue, saying it had never acknowledged having such weapons.
On Monday, Lebanese security officials said Makdissi had flown from Beirut to London. He has not spoken publicly in weeks and it was unclear whether he had left the government.
NATO foreign ministers also met with their Russian counterpart, Sergey Lavrov. The Kremlin has stymied more than a year of international efforts to apply global pressure on the Assad regime, its strongest ally in the Arab world, but officials say it has expressed equal concern about the threat of any chemical weapons.
Speaking to reporters, Lavrov said Russia wouldn't object to the Patriots.
"We are not trying to interfere with Turkey's right" to defend itself, he said. "We are just saying the threat should not be overstated."
Lavrov stressed that Syrian artillery strikes into Turkey were accidental. And he warned that the conflict "is being increasingly militarized."