Powell delivered his warnings Sunday night in a speech to the American Israel Public Affairs Committee, a Jewish lobby group.
"Syria can continue direct support for terrorist groups and the dying regime of Saddam Hussein, or it can embark on a different and more hopeful course," he said. "Either way, Syria has the responsibility for its choices and for the consequences."
Powell also admonished the Iranian leadership, saying, "It is now time for the entire international community to step up and insist that Iran end its support for terrorists, including groups violently opposed to Israel and to the Middle East peace process. Tehran must stop pursuing weapons of mass destruction and the means to deliver them."
In a statement faxed to The Associated Press Monday, the Syrian Foreign Ministry railed against the U.S.-led war in Iraq.
"Syria has chosen to be with international legality represented by the United Nations Security Council. Syria has also chosen to stand by the Iraqi people who are facing an illegal and unjustified invasion and against whom are being committed all sorts of crimes against humanity," the statement said.
"It is clear from [Powell's] speech that he was submitting a report on his latest achievements which confirm that what the U.S. administration is doing in our region serves Israel and its interests and pleases [Israeli Prime Minister Ariel] Sharon," the statement continued.
Powell's warnings followed those Defense Secretary Donald Rumsfeld issued last Friday, accusing Syria of supplying military technology to the Iraqi regime.
"We consider such trafficking as hostile acts and will hold the Syrian government accountable for such shipments," he said. Rumsfeld also said Iraqi Shiite forces in Iran would be dealt with as enemy forces if they intervened in the war with Iraq.
Syrian foreign minister Farouk Al-Sharaa slammed Rumsfeld's criticisms, saying the United States was trying to cover up failures in Iraq. He said the U.S. was killing Iraqis under the guise of liberating the country and protecting democracy, justice and human rights.
Addressing parliament on Saturday, al-Sharaa said the Arab world had reached a "historic crossroad because of the American and British invasion of a brotherly Arab country."
"Where is the interest of the American people in this aggression? And what are the [American] Defense Department's sources on Syria and the region?" he asked.
On Monday, Syrian information minister Adnan Omran, a former Syrian ambassador to Britain, responded to Rumsfeld's allegations that Syria had provided Iraq with war equipment such as night-vision goggles.
"It takes only a madman to widen the circle of war," Omran said. "The Pentagon is in real difficulties. He has to throw blame here and there. If you want to try to maximize the size of your victory, you have to maximize the size of the help your enemy is getting."
Syria, the sole Arab country on the U.N. Security Council, voiced severe disapproval when the U.S. pushed the body to support military strikes in Iraq. Syria-U.S. relations were further strained on March 12, when Powell accused Damascus of developing weapons of mass destruction and called Syrian military presence in Lebanon an "occupation."
Syria is on the U.S. State Department's list of countries supporting terrorism, and Syrian officials say they worry they could be the next target of American strikes after Iraq.
"The possibility is always there," Syrian President Bashar al-Assad told Lebanon's as-Safir newspaper last week. "As long as Israel exists, the threat is there. As long as there is an aggression on an Arab country and a war on our borders, the danger is there."