AKCAKALE, Turkey -- Turkey sanctioned further military action against Syria on Thursday and bombarded targets across the border with artillery for a second day, raising the stakes in a conflict that increasingly is bleeding outside Syrian territory.

Although both sides moved to calm tensions, Turkey's parliament overwhelmingly approved a bill allowing the military to conduct cross-border operations into Syria -- showing it has military options that do not involve its Western or Arab allies.

It was the most dramatic escalation in tensions between the countries, which were close allies before the revolt against Syrian President Bashar Assad began in March 2011.

For the past 18 months, however, Turkey has become one of the strongest critics of the Syrian regime, accusing it of massacres against the opposition.

Rebels trying to bring down Assad have used Turkey as their base, enraging a regime that accuses foreign countries of fomenting the unrest inside Syria.

Sparking the latest hostility was a mortar shell fired from Syria that slammed into a house in the Turkish border village of Akcakale on Wednesday, killing two women and three children.

"(The shell) hit my neighbor next door. His wife, his children died," villager Bakir Kutlugil said. "Now I worry whether the next one will hit me or my neighbor."

Turkish response to the Syrian shelling was swift -- it fired salvos of artillery rounds inside Syria, contacted its NATO allies and convened Parliament for a vote authorizing further cross-border military operations if necessary.

The bill opens the way for action by Turkey's armed forces inside Syria. Turkey has used a similar provision to repeatedly attack suspected Kurdish rebel positions in northern Iraq.

Syria's U.N. envoy said his government was investigating the source of the cross-border shelling and did not want any escalation of violence with Turkey.

The Assad regime, according to ambassador Bashar Ja'afari, has sent its "deepest condolences" to the families of the victims but stopped short of an apology, pending the outcome of an investigation.

Ja'afari urged Turkey to act "wisely, rationally" and prevent infiltration of "terrorists and insurgents" and the smuggling of arms across the border.

Turkish officials characterized the statement as an apology.

Ja'afari said return shelling from Turkey early Thursday injured two Syrian arm officials.

The border violence has added a dangerous new dimension to Syria's civil war, dragging Syria's neighbors deeper into a conflict that activists say has already killed 30,000 people.