Syria escalated tensions with Turkey on Monday, accusing its neighbor and former friend of imperialist delusions harking back to the centuries of Ottoman dynastic rule, as Syrian army gunners exchanged artillery blasts with their Turkish counterparts across the border for the sixth day.

Both insurgent sympathizers and the Syrian government described an extremely violent day in the nearly 19-month-old uprising, with unverified accounts of killings and destruction in the embattled cities of Aleppo, Homs, Daraa and in northern Idlib province, where members of the rebel Free Syrian Army claimed to have discovered a massacre committed by security forces at a makeshift prison.

There also were signs that the Syrian National Council, the main opposition group in exile, was attempting to be more relevant to a future political solution by convening a special conference next week in Doha, Qatar.

In what appeared to be part of that effort, the council's president, Abdulbaset Sieda, was quoted as saying the group would not rule out a future role for any members of President Bashar Assad's government, as long as they had not ordered or participated in killings, which by some estimates have left more than 20,000 Syrians dead.

Syria's information minister, Omran al-Zoubi accused the Turkish government of behaving as though it had reverted to the geopolitics of Ottoman dominance that shaped the Middle East for more than 600 years, before that monarchy was abolished in 1922.

"Turkey isn't the Ottoman Sultanate," Zoubi was quoted as saying. "The Turkish Foreign Ministry doesn't name custodians in Damascus, Mecca, Cairo and Jerusalem."

Turkey and Syria once enjoyed one of the strongest friendships among Middle East neighbors. They grew estranged after Assad's government brutally suppressed opposition that started with peaceful demonstrations in March 2011. Turkey's government has since called Assad unfit to govern.