At NATO headquarters in Brussels, U.S. Secretary of State Hillary Rodham Clinton and the alliance's 27 other foreign ministers "unanimously expressed grave concerns about reports that the Syrian regime may be considering the use of chemical weapons," according to Fogh Rasmussen.
"Any such action would be completely unacceptable and a clear breach of international law," he said.
His comments came a day after President Barack Obama warned of consequences if Assad made the "tragic mistake" of deploying chemical weapons. American officials say the U.S. and its allies are weighing military options in light of intelligence reports showing the Syrian regime may be readying its unconventional weapons and may be desperate enough to use them.
German ambassador Martin Erdmann said the Bundestag will probably take up the matter next week.
The decision was announced after the NATO foreign ministers met Tuesday 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," and that more weaponry in the area would only add to that problem.
Addressing Lavrov and the other 27 NATO foreign ministers, Clinton said Washington and Moscow still have major differences on the political transition needed in Syria. She and her NATO partners issued a statement later, also stressing that the Patriots "will in no way support a no-fly zone or any offensive operation."
Turkey, one of the harshest critics of the Assad regime, asked for the Patriots to defend against possible retaliatory attacks by Syrian missiles. It welcomed the NATO decision, adding in a government statement that it would press on with efforts "to solve the Syrian crisis through peaceful ways, with the same resolve as before."
Turkey and Syria share a porous, 566-mile (911-kilometer) border, which has allowed rebel leaders to take shelter in Turkey and brought the countries to near war in recent months. Syria was blamed for shooting down a Turkish plane and for lobbing mortars that killed two women and three children.
Syria, which is party to the 1925 Geneva Protocol banning chemical weapons in war, has repeatedly insisted it would not use them even if it did possess such weapons.
Associated Press writer Christopher Torchia in Istanbul contributed to this report.