In yet another sign that military action may be imminent, Kerry made phone calls on Friday to his counterparts in Europe and the Persian Gulf, as well as the head of the Arab League.
Russian President Vladimir Putin today rejected the American evidence that Syrian forces used chemical weapons, calling the suggestion "utter nonsense."
"While the Syrian army is on the offensive, saying that it is the Syrian government that used chemical weapons is utter nonsense," Putin told journalists in Vladivostok, according to Russia's Interfax news agency.
He urged the United States to present its evidence to the U.N. Security Council and cautioned Obama about the consequences of getting involved.
"I would like to address Obama as a Nobel Peace Prize laureate: before using force in Syria, it would be good to think about future casualties," he said.
Syria and Its Neighbors Brace for Possible Strike
Obama's speech was broadcast live on Syrian state TV today, and Syrians are bracing for an imminent attack. In many parts of Damascus, normal life has largely continued amid the constant din of shelling in the suburbs, but the prospect of a U.S. military strike has ushered in a widespread sense of unease.
"We are not afraid of death any more, we are awaiting it," Amal, a Damascus resident told the BBC. "We just need an end to all of this."
Journalists on the ground in the capital report the city is tense but not panicked; cafes and shops are still open and many Damascus residents are stocking up on staples.
Syrian Prime Minister Wael al-Halqi said today that "the Syrian army is fully ready, its finger on the trigger to face any challenge or scenario that they want to carry out."
Citing Syrian security sources, Al Mayadeen, the Lebanese TV station affiliated with the Shi'ite militant group Hezbollah and with close ties to the Assad regime, reported that the Syrian government was expecting an attack at any moment.
As the government awaits a U.S. strike, the Free Syrian Army (FSA) and its affiliates are standing by for the regime to lash out following a strike. Members of the opposition who spoke with ABC News largely believe a U.S. strike would be wider-ranging than the Obama administration has described.
The nature of the U.S. strike will determine the Syrian government's reaction and the opposition's next move.
According to opposition sources, FSA commanders in northern Syria have told their fighters that the regime will immediately start attacking rebel-controlled positions.
The same commanders also advise the chaos will provide a good opportunity to cut off supply lines, target supply lines and capture supplies.
But regardless of the regime's response, both the FSA and the Israeli Defense Forces (IDF) across the southern border expect Hezbollah to respond in some form.
The FSA is ready to defend against an onslaught of militants on the ground, and the IDF has reinforced its missile defenses in the north.
On Friday, the IDF deployed an Iron Dome battery in the Tel Aviv area facing north. Iron Dome batteries were also deployed in Haifa, Ashkelon, and Eilat and additional batteries are ready to be moved as needed, according to Israeli media. Last week, Israel ordered a limited call-up of reserve troops.
But even as the IDF takes precautionary steps and nervous Israelis wait in long lines for free gas masks, there is no widespread panic in Israel.
The newspaper Israel Hayom released a poll this week that found two-thirds of Israeli Jews are in favor of a military intervention in Syria, but most also expected a retaliation against Israel.
Seeking to reassure Israelis, Prime Minister Benjamin Netanyahu said, "There is no reason to change daily routines."
"We are prepared for any scenario," he said.
ABC News' Dana Hughes reported from Washington, Alex Marquardt and Nasser Atta contributed reporting from Beirut, Ben Waldron reported from UN Headquarters and Kirit Radia reported from Moscow.