Activists say more than 150 people were killed in Thursday's attack, which they allege was carried out by the army, backed by pro-regime militiamen known as shabiha ("ghosts" in Arabic).
Syria's military, however, said the army had killed "many terrorists" in Treimsa, but no civilians, in a "special operation... targeting armed terrorist groups and their leadership hide-outs."
Ghosheh said a "wide range of weapons were used, including artillery, mortars and small arms."
"The attack on Treimsa appeared targeted at specific groups and houses, mainly of army defectors and activists. There were pools of blood and blood spatters in rooms of several homes together with bullet cases.
"The UN team also observed a burned school and damaged houses with signs of internal burning in five of them."
The number of casualties was still unclear, she added.
The Treimsa killings have triggered a global outcry against the regime of President Bashar al-Assad, with UN chief Ban Ki-moon calling for urgent action to stop the bloodshed.
The head of Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP it "might be the biggest massacre committed in Syria since the start of the revolution" against Assad in March 2011.
If confirmed, the 150-plus toll would exceed that of a massacre at Houla on May 25, when a pro-Assad militia and government forces were accused of killing at least 108 people.
Ghosheh said the observers planned to return to Treimsa on Sunday for further investigations.
"UNSMIS is deeply concerned about the escalating level of violence in Syria and calls on the government to cease the use of heavy weapons on population centres and on the parties to put down their weapons and choose the path of non-violence for the welfare of the Syrian people who have suffered enough," she said.
The Observatory said earlier that Syrian troops and pro-regime militias had stormed and torched a town in southern Syria on Saturday.
Hundreds of soldiers backed by helicopter gunships attacked Khirbet Ghazaleh in the province of Daraa -- the cradle of a 16-month uprising -- amid heavy gunfire, the watchdog said.
An activist on the ground who identified himself as Bayan Ahmad gave a similar account, saying pro-regime militias has set alight houses in the town.
"The army entered without resistance as the rebel Free Syrian Army left town. The shelling has wounded dozens of people but we don't have medical resources to treat them," he added.
Elsewhere, a pregnant woman was among 72 people killed across the country on Saturday, the Observatory said, a day after 118 people died including dozens of civilians gunned down by troops at anti-regime protests.
Those killed were 34 civilians -- including nine women and seven children -- 17 rebels and at least 21 soldiers, it said.
An AFP journalist said fighting Saturday near the Turkish border between government troops and rebel fighters had left at least 10 rebels dead and 15 wounded.
Treimsa is near Al-Kubeir, where at least 55 people were killed on July 6, according to the Observatory. Like Al-Kubeir, Treimsa is a majority Sunni village situated near Alawite hamlets.
Assad belongs to the Alawite community -- an offshoot of Shiite Islam -- although most Syrians are Sunni.
UN chief Ban Ki-moon lashed out at the Syrian regime and called for the UN Security Council to urgently act to stop the bloodshed, as failing to do so would give "a licence for further massacres."
The Treimsa killings have added urgency to deadlocked Security Council negotiations on a Syria resolution.
The US ambassador to the United Nations, Susan Rice, said on Twitter that the killings "dramatically illustrate the need for binding measures on Syria" by the council.
Western nations have proposed a resolution that would impose sanctions on the regime of Syrian President Bashar al-Assad over the conflict, which rights activists say has cost more than 17,000 lives.
Britain, France, the United States, Germany and Portugal have proposed a resolution that would give Assad 10 days to stop the use of heavy weapons, in line with the Annan plan, or face sanctions.
They also want to give the UN observer mission a new mandate, but for only 45 days. Their mandate ends on July 20.
Russia has rejected as unacceptable any use of sanctions. It is proposing a rival resolution that renews the mandate of UNSMIS for 90 days.