The team of inspectors has been in Damascus since Monday carrying out investigations into previous claims of chemical weapons use during the country's civil war.
However, the Assad regime has yet to allow them to investigate the area where opponents claim his forces gassed more than 1,300 people to death on Tuesday, many of them children.
Activist Abu Nidal said: "The UN team spoke with us and since then we prepared samples of hair, skin and blood and smuggled them back into Damascus with trusted couriers."
However, other activists said the couriers may have struggled to get through to the inspectors who are under heavy guard and government minders.
President Bashar al Assad's forces have been pummelling the area of the attack, known as the Ghouta region, with air raids and artillery strikes, in a move which could hinder access and damage evidence.
The development came as US President Barack Obama made his first comments on the attack saying that images and claims indicated a "big event of grave concern" and warranted "America's attention".
But he said that evidence had to be gathered and that while people looked to the US to act, he could not do so without a UN mandate.
The UN High Commissioner for Human Rights Navi Pillay has said that her staff members have found confirmations of numerous deaths and thousands of wounded in the course of a military operation in the Damascus suburb of Guta this past Wednesday.
Navi Pillay strongly condemned the reported killing of hundreds of civilians – including many women and children - in the Damascus suburbs, and said the allegations that many deaths were the result of the use of chemical weapons must be investigated “as a matter of utmost urgency.”
"The Secretary-General’s team, headed by Dr. Sellstrom, is currently in Syria," Pillay said. "I urge the Government and opposition forces to enable them to examine the site of the alleged attacks without any delay or obfuscation."
"The use of chemical weapons is prohibited under customary international law,' the High Commissioner said. “This absolute prohibition applies in all circumstances, including armed conflict. As a norm of customary international law, it is binding on the Government despite it not being party to the 1993 Chemical Weapons Convention. It is also binding on anti-Government armed groups.'
'These allegations are exceptionally grave and need to be comprehensively proved or disproved as soon as is humanly possible," she added. "Whether or not chemical weapons were in fact used, it seems that once again in Syria many civilians have been killed in flagrant contravention of international law."
Pillay said her staff in the region had managed to contact well-placed sources inside Syria who report that, in addition to those killed, there are thousands of injured people in desperate need of medical care and humanitarian aid. Parts of the affected suburb of Al Ghouta were already suffering from protracted and severe shortages of supplies before the events of recent days.
"I understand shelling and fighting have continued today," Pillay said. "I call on all parties to halt the fighting immediately and allow access to humanitarian aid and essential medical supplies, in order to prevent more needless deaths."
Voice of Russia, AFP, bloomberg.com, RIA, Reuters, dpa, RT, Interfax, Reuters, Sky News