The continued exchange of artillery fire between Syria and Turkey raises additional concerns that the conflict may escalate and spread to neighbouring countries, US Defence Secretary Leon Panetta has said.
Mr Panetta said the US is using its diplomatic channels to relay worries about the fighting in the hopes that it will not broaden.
His comments came on the heels of warnings from Turkey's prime minister that his country is not far from war with Syria.
Turkish and Syria traded artillery fire on Saturday as rebels clashed with President Bashar Assad's forces near the border, heightening the fears that the crisis could erupt into a regional conflict. Turkish Prime Minister Recep Tayyip Erdogan on Friday cautioned Damascus not to test Turkey's "limits and determination" and said Ankara was not bluffing in saying it will not tolerate such acts.
Saturday's cross-border exchange began when two mortar shells fired from Syria landed in rural areas near the Turkish village of Guvecci, prompting Turkish return fire, Turkey's media reported.
Later, a third shell hit near another village in Turkey's Hatay province and Turkish troops fired back, the office of the provincial governor said. No casualties were reported.
Relations between Turkey and Syria, once strong allies, deteriorated sharply after the uprising against Assad began in March last year. Turkey became one of the harshest critics of Assad's crackdown while Syria accused Ankara of aiding rebels.
Turkish foreign minister Ahmet Davutoglu told Turkey's state television TRT that "from now on whenever there is an attack on Turkey, it will be silenced".
Also on Saturday, Assad made a rare public appearance when he laid a wreath at the country's Unknown Soldier statue in Damascus to mark the anniversary of the 1973 war with Israel, also known in Syria as the October War. Syrian state television broadcast the ceremony and likened the current crisis to the war with Israel.
Damascus denies it is facing a popular uprising, instead blaming the violence on a foreign conspiracy linked to its support for anti-Israeli groups such as Lebanon's Hezbollah.