Syria's military said Thursday it had captured a string of towns and part of a key highway from the Islamic State (IS) militant group in the northern province of Aleppo.

Since launching a broad offensive against the Islamist militants nearly three weeks ago, the army has taken around 250 square kilometres (100 square miles) of territory, it said in a statement carried by official news agency SANA.

It said the area includes more than 32 towns and fields.

"This achievement widens the security perimeter around Aleppo city and forms a launching pad for developing military operations against Daesh," the army said, using an Arabic acronym for IS.

Forces loyal to President Bashar al-Assad have steadily advanced in Syria's north since announcing they had retaken full control over Aleppo city in late December.

It was the biggest blow to Syria's rebel movement since the uprising against Assad erupted in March 2011.

Syria's army on Thursday also said it had seized a 16 kilometre-stretch (10 miles) of highway linking Aleppo city to the IS-held town of Al-Bab.

Al-Bab, 25 kilometres (15 miles) south of the Turkish border, is the last remaining IS bastion in Aleppo province and is seen as a prize by nearly all sides of Syria's complex war.

Since December, Turkey-backed rebel fighters known as the Euphrates Shield alliance have edged towards Al-Bab from the north.

Turkish President Recep Tayyip Erdogan pledged last month that "the operation in Syria's Al-Bab will be finished in a short time, God willing."

But Ankara's involvement has angered Damascus, which on Thursday said it delivered two letters to the United Nations Security Council rebuking Turkey's military operations in Syria.

"In recent days, Turkish armed forces have carried out an incursion into Syrian territory and have occupied some Syrian villages, including two west of Al-Bab," said the letters, sent by the foreign ministry and published by SANA.

"This represents a flagrant violation of Syrian sovereignty and of the principles and purposes of the UN Charter and the rules of international law," they said.

The letters accused Ankara of supporting "terrorism" and said the Security Council should act to "put an end to the violations of the Turkish regime".

In January, Turkey's air force began carrying out joint bombing raids around Al-Bab with Assad ally Russia.

The two parties back opposing sides in the war but have joined forces in recent months to try to bring an end to the bloody conflict.

More than 310,000 people have been killed in Syria and millions have been displaced since the uprising broke out nearly six years ago.

One person was killed on Thursday in shelling by regime forces and their Lebanese ally Hezbollah near Damascus, according to the Syrian Observatory for Human Rights.

The Britain-based monitor said "dozens" of rounds hit Madaya and Baqin, two towns west of Damascus that had been under government siege for months before a local reconciliation deal.

They wounded several people and killed at least one person in Baqin, the Observatory said.