Russia has sent sophisticated anti-ship missiles to Syria, US media report.
The New York Times quotes unnamed US officials as saying the missiles could be used to counter any potential future foreign military intervention in Syria.
Without confirming details, Russia's foreign minister said Russian supplies did not break any international rules.
It comes amid growing alarm that chemical weapons may be being used in Syria, something US President Barack Obama has said would be "a red line".
Meanwhile efforts continue to arrange an international conference on Syria.
The United Nations Secretary General Ban Ki-moon met Mr Lavrov in Sochi on Friday to discuss the plans for the conference, which would aim to bring together the Syrian opposition and members of President Bashar al-Assad's government.
At a news briefing, Mr Ban said it was important to "not lose momentum" on the drive towards holding a peace conference and dates for it were being "actively discussed".
Mr Lavrov said a resolution could only be found through "an inclusive all-Syrian dialogue with participation of all Syrian forces, without any external intervention, as soon as possible".
Also on Friday, the UN's refugee agency said more than 1.5m Syrians were now registered as refugees, with the true figure likely to be much higher.
"Refugees tell us the increased fighting and changing of control of towns and villages, in particular in conflict areas, results in more and more civilians deciding to leave," UNHCR said in a statement.
'Ship-killers'
Russia is one of Syria's few remaining allies and its major arms suppliers. Over the years, in contracts worth billions of dollars, it has sold thousands of tanks, artillery units, aircraft, helicopters and defence systems to Damascus.
According to the New York Times report, a recent Russian shipment to Syria included an advanced form of the Yakhont, a 6.7m-long (22ft) missile with a range of 290km (180 miles) and carrying either a high-explosive or armour-piercing warhead.
The initial order - for 72 missiles along with launcher and support vehicles - was placed in 2007 and the first deliveries received in early 2011, said the paper. It quotes two unnamed senior US defence officials as saying the most recent shipments had more advanced radar guidance systems, enabling it to evade a ship's defences.
Sergei Lavrov said on Friday he did not understand "why the media is trying to create a sensation out of this".
"We have not hidden that we supply weapons to Syria under signed contracts, without violating any international agreements, or our own legislation.
"And we most importantly supply anti-aircraft system, and it doesn't create any imbalance of power in the region or any kind of advantages in the fight against the opposition."
Another US newspaper, the Wall Street Journal, reports that Moscow has deployed at least a dozen warships to patrol waters near the Russian naval base in the Syrian city of Tartus.
It quotes a senior US defence official as saying the deployment is a "show of force" by Moscow to demonstrate its commitment to the region.
Although there have been growing calls for arms to be channelled to the rebel fighters in Syria, there has so far been very limited enthusiasm in the West for outright military intervention.
But there is concern that the presence of sophisticated Russian-supplied weaponry will make it much harder to agree and carry out such intervention, implement a blockade or conduct targeted airstrikes in the future.
Nick Brown, editor-in-chief of the influential military journal Jane's International Defence Review, said the Yakhont was "a real ship killer".
"It enables the regime to deter foreign forces looking to supply the opposition from the sea, or from undertaking a more active role if a no-fly zone or a shipping embargo were to be declared at some point" he told the Times.
Israel is also concerned such weapons could fall into the hands of Lebanese militant group Hezbollah, which could use them to either attack Israel or defend itself against any Israeli assault.
During his visit to Moscow on Tuesday, Israeli Prime Minister Benjamin Netanyahu reportedly discussed the issue with Mr Putin, while Israeli jets have carried out strikes on Syrian targets to block alleged transfers of weapons to Hezbollah.
Despite its arms sales, Russia has hosted several world leaders in recent weeks in an attempt to find a way of ending the Syrian conflict, which has left an estimated 80,000 people dead.
The BBC's Steve Rosenberg is Moscow says the fact that Mr Ban, Mr Netanyahu, US Secretary of State John Kerry and UK Prime Minister David Cameron have travelled to Russia for talks shows they believe Moscow is the key to ending the crisis.
The Yakhont is a radar-guided, supersonic anti-shipping missile designed for coastal defence. Depending upon its trajectory it has a range of between 120 and 300 km (75-186 miles).
US reports stress the ability this gives to the Assad government to push hostile naval forces away from Syria's shores.
But the real significance of these reports, if true, is twofold.
Firstly they show Russia's continuing desire to ensure that there be no Western intervention in Syria along the lines of what happened in Libya.
But equally the real concern about the Yakhont may be their potential transfer to Syria's Lebanese ally Hezbollah. Israel has made it clear that a transfer of the Yakhont represents a red line that could prompt more air strikes against Syrian arms depots.
Analysis
US reports stress the ability this gives to the Assad government to push hostile naval forces away from Syria's shores.
But the real significance of these reports, if true, is twofold.
Firstly they show Russia's continuing desire to ensure that there be no Western intervention in Syria along the lines of what happened in Libya.
Russian naval deployments in the Mediterranean back up this view.
But equally the real concern about the Yakhont may be their potential transfer to Syria's Lebanese ally Hezbollah. Israel has made it clear that a transfer of the Yakhont represents a red line that could prompt more air strikes against Syrian arms depots.
