(CNN) -- Undeterred by Syria's gruesome war of attrition, rebel fighters have scored a key gain where government forces once ruled. The rebels have choked off a sprawling military base outside Aleppo with some 450 government soldiers trapped inside. The rebels could easily overrun the base, fighter Ali Jadlan said. But they want to give government soldiers a chance to defect. Already, about 250 soldiers have escaped the siege, and most of them have joined the opposition. It's another indication President Bashar al-Assad is losing his grip on a country he once firmly commanded. The government has tried air-dropping food to its soldiers, often missing its targets. Opposition fighters have shot out their water supply. While the soldiers still have stockpiles of artillery, their options are dwindling. "They have reached a point where they think that they can't go back," said Jamal, a defected soldier. "They have reached a dead end. Slowly, they are weakening." But the government isn't giving up. Fierce artillery and missile shelling bombarded the opposition hotbed of Homs for the 35th straight day, dissidents said Tuesday, as government and rebel forces battled in neighborhood streets. The renewed violence comes a day after 239 people were killed across Syria, said the opposition Local Coordination Committees of Syria. Fears of chemical weapons U.S. officials are concerned that with fighting closing in on Syria's capital of Damascus, the government may be toying with the idea of using chemical weapons to crush the 21-month rebellion. U.S. President Barack Obama issued a stern warning to al-Assad's government: "The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences, and you will be held accountable," he said. The Syrian Foreign Ministry denied that the country had any plans to use chemical weapons, state TV reported. The government has repeatedly stressed it will not use such weapons, if they exist, against its people under any circumstances. But U.S. officials say "worrying signs" suggest otherwise. Syrian forces have started combining chemicals that could be used to make deadly sarin gas for weapons to attack rebel and civilian populations, one U.S. official said. The intelligence, obtained over the weekend, the official said, came from multiple sources. But the official declined to provide more details about how the United States learned of it. Sarin gas, the source said, could most readily be used to fill artillery shells. Bracing for the worst in Turkey The Syrian civil war has spilled into Turkey, where errant Syrian artillery shells struck the border town of Akcakale and killed five Turkish civilians in October. Turkey has asked NATO for Patriot missiles to bolster its air defenses against Syrian threats. NATO is expected to approve the request Tuesday. The United States, Germany and the Netherlands, which all have Patriot capabilities, have signaled they may be willing to contribute missiles should NATO approve the deployment to Turkey. NATO Secretary General Anders Fogh Rasmussen stressed the possible deployment of Patriot missiles would be "purely defensive," and not aimed at preparing a no-fly zone. "We have no intention to intervene militarily in Syria. We will do what it takes to protect our ally Turkey," he said. But for residents on the Turkish side of the border, fears escalate almost every day. Warplanes bombed a Syrian town within sight of the Turkish border Monday, killing at least 10 people, opposition activists say. Thick black smoke rose from the embattled town of Ras al-Ain. Across the border in Turkey, Dr. Feras Haj Khbbur described the carnage among rebel fighters and civilians. "The injuries are bad. Big injuries. Lots of arms and legs are gone."
