The head of a prominent Syrian opposition group says U.S. President Barack Obama should take greater action on Syria and not be worried about whether the "right decision" will hurt Obama's re-election campaign.

The plea comes after more than 200 people were reportedly massacred in the Syrian town of Tremseh -- the deadliest day in Syria's 16-month crisis -- and as activists reported the heaviest clashes yet in Damascus.

"We want for America and the Western countries to carry out their responsibilities through the (U.N.) Security Council and work to adopt a resolution under Article 7 to force this regime to stop killing Syrians," Abdulbaset Sieda, chairman of the Syrian National Council, said in an interview that aired Sunday on CNN's Fareed Zakaria GPS.

"With regard to America, specifically, we would like to say to President Obama that waiting for election day to make the right decision on Syria is unacceptable for the Syrians. We cannot understand that a superpower ignores the killing of tens of thousands of Syrian civilians because of an election campaign that a president may win or lose. That's why we are saying there is work that must take place at the Security Council."

The Obama administration has decried the Syrian regime and called for Syrian President Bashar al-Assad to step down. But previous attempts to pass resolutions condemning the regime at the U.N. Security Council have failed due to vetoes from Russia and China.

If continued efforts at the Security Council fail, the United States and other countries "can move outside the scope of the Security Council," Sieda said.

At least 72 people were killed Sunday in cities across the country, according to the opposition Local Coordination Committees of Syria (LCC).

The deaths included 11 people "martyred under torture in the regime's prisons," seven soldiers who defected, three women and six children, the group said.

There were deaths in Homs, Deir Ezzor, Aleppo, Daraa, Idlib, Hama, Swaida, Damascus suburbs, and Damascus itself, the LCC said.

Activists reported violence in various parts of the capital on Sunday.

There was shelling and clashes in the neighborhood of Tadamon in southeastern Damascus, said Abu Qais, a resident.

"Today is the first day tanks enter this part of the capital," he said, adding that he could see a "mass number" -- at least 20. Also, "hundreds of members of the security forces" were storming the neighborhood of mixed sects, he said.

The rebel Free Syrian Army took some positions in the neighborhood two weeks ago and there have been sporadic clashes, "but this is the first time the security forces are able to defeat the defenses of the FSA positions and storm the neighborhood," he said.

"We have at least 80 people injured, amassed in a small field clinic. There are probably dead people but I have no idea how many. This appears to be another Baba Amr," he said, referring to a town in which residents died under government siege earlier this year.

The opposition LCC reported that regime forces entered the neighborhood with tanks as heavy clashes were taking place.

The group reported "dead bodies on the streets as a result of the violent shelling at the neighborhood."

The Damascus International Airport highway and all of its exits were closed, the LCC said.

The towns of Aqraba and Sidi Kadad, as well as some Palestinian refugee camps, were surrounded by tanks and armored vehicles, and heavy shelling was reported, the LCC said.

The opposition Syrian Observatory for Human Rights reported that the neighborhood was shelled with artillery fire before the tanks went in.

The observatory also reported shelling and the sounds of heavy gunfire in al-Hajar al-Aswad and al-Qurshi, two areas close to Tadamon.

Sunday's clashes included "the most intense in the capital so far" since the uprising began last year, the observatory said.

Throughout the conflict, al-Assad's regime has consistently blamed the violence on "armed terrorist groups," and reported on its security forces "martyred" in attacks.

U.N. observers returned to Tremseh on Sunday.

A patrol found more than 50 houses burned or destroyed, the U.N. Supervision Mission in Syria said in a statement.

It added gruesome detail: "Pools of blood and brain matter were observed in a number of homes."