Syrian rebels fighting President Bashar al-Assad's troops won a fresh boost when the European Union eased its oil embargo to let them exploit the resources they control.
But EU foreign ministers meeting in Luxembourg stopped short of lifting its arms embargo.
Their decision came ahead of a meeting Tuesday between Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry in Brussels.
Already Monday, Russia, one of Assad's few remaining backers, had criticised the EU measure as "counter-productive".
"This deepens the impasse and does not contribute to a political solution to problems which have built up over a long time," vice-foreign minister Mikhail Bogdanov told the Ria Novosti news agency.
Such unilateral action violated international law, he added.
The new measures allow European companies to import Syrian crude and export oil production technology and investment cash to areas held by the rebel forces.
"We want regions controlled by the opposition to develop, we want to help economic reconstruction," said German Foreign Minister Guido Westerwelle.
The decision to maintain a ban on military aid echoed the stance taken by Washington this weekend. At a "Friends of Syria" in Istanbul, Kerry refused to arm the opposition but doubled US non-military assistance to $250 million.
The EU recently eased its arms embargo to allow the supply of "non-lethal" equipment as well as "technical assistance" -- which includes training -- to the rebels.
Britain and France have argued that the bloc should go further and lift the ban on weapons shipments to the rebels, but that would require unanimous support.
This weekend Syrian opposition chief Ahmad Moaz al-Khatib's stepped down in protest at world "inaction" over the fighting in Syria, which has left more than 70,000 people dead.
The White House said Monday it was appalled by "horrific" reports of a recent massacre in Syria after a watchdog group said 101 people, many of them civilians, had been killed in Jdaidet al-Fadl, a town near Damascus.
Violence across Syria on Monday killed at least 95 people, including 29 civilians, 33 rebel fighters and 33 regime soldiers, according to the Syrian Observatory for Human Rights.
On the ground in Syria, Assad's forces have made gains in recent days in the Damascus region and in the central province of Homs.
Signup to our newsletter and follow us on Facebook and Twitter!
The regime's capture of several villages in the Homs province has raised fears among rebels that the town of Qusayr -- an opposition stronghold -- could also fall.
The Britain-based Observatory told AFP that elite fighters from the Lebanese Shiite group Hezbollah, a staunch ally of the Syrian regime, were leading the fight against rebels there.
George Sabra, the interim chief of the opposition National Coalition, said Monday that Hezbollah's role in the fighting around Homs, near the border with Lebanon, amounted to a "declaration of war".
Lebanon's official news agency quoted senior Hezbollah leader Sheikh Nabil Qauk as saying: "What Hezbollah is doing with regard to this issue is a national and moral duty in the defence of the Lebanese in border villages."
In Tel Aviv, Defence Minister Moshe Yaalon said Israel had stopped advanced weapons from falling into militant hands, having made it clear to Damascus that it could not let such arms go to Hezbollah "or other rogue elements".
"When they crossed this red line, we acted," he said.
This was seen as implicit confirmation of Israeli involvement in a January 30 strike on an arms convoy inside Syria, which hit what a US official has said were surface-to-air missiles near Damascus.
Yaalon said Israel had also set out two other "red-line" issues to Damascus: security along the Israeli-Syrian ceasefire line on the occupied Golan Heights; and any transfer of chemical weapons into the hands of militants.
UN leader Ban Ki-moon appealed Monday for an end to outside arms supplies to rival sides during talks with Arab League Secretary General Nabil al-Arabi and Qatari Prime Minister Sheikh Hamad bin Jassem Al-Thani.
Qatar has been accused by Damascus of arming Syrian rebels.
But Al-Arabi rejected the idea in comments later to reporters in New York. Halting arms supplies would only be possible if there was at least the beginning of a political settlement, he said.
"The government is getting arms from certain parties so if the other side gets some arms from certain parties I think you can get some form of a balance there," he added.
Syria's state news agency SANA reported late on Monday that an armed group had kidnapped two bishops in a village in Aleppo province in northern Syria.
It named them as Bishop Yohanna Ibrahim, head of the Syrian Orthodox Church in Aleppo and Bishop Boulos Yaziji, head of the Greek Orthodox church there.
The Greek Orthodox diocese of Aleppo declined to comment when contacted by AFP, but Christian residents in Aleppo said gunmen had intercepted the two bishops' car, killing their driver.