Islamic State fighters stormed the Syrian town of Tel Abyad on the Turkish border on Tuesday and captured a neighbourhood from the Syrian Kurdish YPG militia, but suffered setbacks in the northeastern city of Hasaka, a monitor and the army said.
Islamic State has launched simultaneous attacks against Syrian government and Kurdish militia this past week in the multi-sided Syrian civil war after losing ground to Kurdish-led forces near the capital of its "caliphate".
Backed by US-led air strikes, the YPG militia has advanced deep into the militants' stronghold province, Raqqa, capturing key positions from the jihadists, including Tel Abyad.
The Syrian Observatory for Human Rights monitoring group said there were heavy clashes on Tuesday around Tel Abyad, which serves as a border crossing, and that militants had taken control of an area in the eastern part of the town.
YPG spokesman Redur Xelil said clashes were ongoing with the militants who had infiltrated a village on the outskirts of the town. "They are encircled ... this will be mass suicide for them," he told Reuters.
The British-based Observatory, which tracks violence across Syria through sources on the ground, said Islamic State had deployed scores of fighters across several villages.
The ultra-radical group, which holds large tracts of Syria's thinly populated east, launched a lighting assault on the strategic northeastern city of Hasaka last Thursday in a bid to capture government-held districts.
But in the last two days, the Syrian army has been able to regain most of the areas of the city the militants had seized.
The Syrian army's ability to hold on to Hasaka and also separately repel a major rebel assault on the provincial capital of the southern province of Deraa on the border with Jordan, stands in stark contrast to a string of recent setbacks.
Hasaka is important for all sides, because it sits between Islamic State-held territory in Syria and in nearby Iraq.
Syrian state television said in a newsflash the army had retaken most of Ghwyran district, the largest populated area in the city, and one of several districts that were recently overrun by insurgents.
In the Aziziya neighbourhood, further east of Hasaka, the army had gained the upper hand with help from Kurdish-led forces, residents said. On Monday, the army said it took back most of Nashwa district.
The Islamist insurgents have deployed scores of suicide bombers against army checkpoints in Hasaka, enabling them to move into positions deeper inside the city.
Hasaka has been divided into areas run separately by Syrian President Bashar al-Assad's government and regional Kurdish authorities, and has an ethnically and religiously mixed population of Arabs and Kurds.
Although the militants had been mainly driven out, fighting continued in some streets where they had taken up sniper positions, locals said.
"Islamic State had withdrawn from most of the districts but did not leave it completely and their snipers in some areas are preventing the full advance of the army," said Ali Hreith, a Hasaka resident on the Turkish-Syrian border.
State television said the army had destroyed a fire engine laden with explosives that the militants had sought to ram into an army post inside the city's Nashwa district.