Valerie Amos, the UN humanitarian chief, has begun talks in Syria on ways to increase humanitarian aid to civilians trapped or displaced by intensifying fighting between government forces and rebels.
Amos on Tuesday visited schools in Damascus used as shelters for displaced families and met government officials, at the start of a three-day visit to both Syria and Lebanon.
Among those she met were Syria's new Prime Minister Wael al-Halqi, who replaced Riad Hijab following his defection last week, and Ali Haidar, state minister for national reconciliation.
The president of the Syrian Arab Red Crescent, Abdul Rahman Attar, accompanied her to the shelters and meetings with those affected by the conflict.
Amos said events in the country since her last visit in March had left a visible impact.
"Clearly the situation has got worse since I was here in March," she said.
Her visit "aims to draw attention to the deteriorating humanitarian situation in Syria and the impact of the conflict on people either remaining in Syria and who have fled to other countries, including Lebanon", the UN office for the co-ordination of humanitarian affairs said.
Spokesman Jens Laerke said Amos would "express her grave, grave concern" over the situation.
"She will look at the situation on the ground and discuss with the government and humanitarian partners how to scale up the response in Syria," he said.
Aid efforts hindered
UN efforts to launch a significant aid operation in recent months have been hindered by insecurity and Syrian bureaucracy.
UN distribution networks are functioning, but a UN humanitarian appeal of $180m for Syria this year is only 40 per cent funded so far, Laerke said.
Amos, the under-secretary general for humanitarian affairs, will travel to Lebanon on Thursday to meet families who have fled from Syria and liaise with the government and humanitarian agencies.
Amos' visit comes as fighting intensified in Syria's two main cities, Damascus and Aleppo, on Tuesday.
Mazen Ibrahim, an Al Jazeera correspondent in Aleppo, said heavy fighting between government forces and rebels raged in several districts.
"Government forces tried to overtake a new area in Saleheddin but Free Syrian Army fighters stopped them," he said, adding that the regime forces then used air force to bombard Salaheddin, Saif al-Dawla and Shaar.
"In Shaar, four people were killed and more than 20 civilians injured after a MiG-21 jet targeted a street with two missiles," he said.
"During my stay in the area, I saw many people, maybe hundreds, were trying to escape to go inside Aleppo searching for a safe zone to stay in."
Syrian Red Crescent volunteers are delivering aid to thousands of displaced in Aleppo and outlying rural areas, many of them staying in public buildings including schools, the International Committee of the Red Cross said.
"In Aleppo the situation is extremely tense and the fighting continues in several neighbourhoods," ICRC spokesman Hicham Hassan said.
One million displaced
Two million people are now estimated to have been affected by the crisis in Syria and more than one million are internally displaced, the UN said.
The UN refugee agency said on Tuesday that almost 157,600 Syrians had also fled the violence and crossed into Lebanon, Jordan, Turkey and Iraq, although many more have left but not registered officially.
More than 20,000 people have been killed across Syria since the anti-regime revolt broke out in March 2011, according to opposition activist groups.
In-depth coverage of escalating violence across Syria
With President Bashar al-Assad's government facing mounting diplomatic pressure, a top presidential aide has been dispatched to China, which has called for an immediate ceasefire and political dialogue to end the brutal 17-month conflict.
Bouthaina Shaaban is to hold talks with Foreign Minister Yang Jiechi and other officials, the foreign ministry said, adding that Beijing was also considering inviting members of the Syrian opposition to visit soon.
Meanwhile, Muslim leaders gathered in Saudi Arabia on Tuesday to mull proposals to suspend Syria from the 57-member Organisation of Islamic Co-operation (OIC), an action which is strongly opposed by Iran.
The OIC represents 1.5 billion Muslims worldwide and the choice of Mecca, the holiest Islamic city, is considered of particular significance.
Foreign ministers who held a preparatory meeting on Monday recommended the suspension of Syria's membership, Ekmeleddin Ihsanoglu, the OIC chief, said.