(CNN) Iran's role in the Syrian civil war is getting more significant, with Iranian officials headed to peace talks while Tehran boosts its military power in the war-torn country.

Iranian Foreign Minister Javad Zarif and two of his deputies will attend Syrian talks in Austria on Friday, the official Iranian news agency IRIB said.

The United States had invited Iran to join the talks, the Russian Foreign Ministry said Wednesday.

U.S. officials did not immediately comment on the Russian statement.

The meetings will also include Russia, Turkey, Iraq, Lebanon, Egypt and Saudi Arabia.

Iran increases aid to Syrian regime

Iran is increasing its military presence in Syria , a top commander told Iranian TV on Monday.

Brig. Gen. Hossein Salami, deputy commander of the Islamic Revolutionary Guards Corps, said Iran is increasing both the quality and quantity of its presence in Syria, according to Iranian media.

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He described their mission as an advisory role to help the Syrian army loyal to President Bashar al-Assad

Salami said the Iranian officers were providing tactical help for Syrian commanders of battalions in direct battles, as well as weapons and ordnance, operational assistance and help with strategic planning, the Tasnim News Agency reported.

He also conceded that the number of Iranian casualties in Syria had increased.

Earlier this month, Iran announced that a high-ranking general, Hossein Hamedani of the Revolutionary Guards, was killed by ISIS fighters near Aleppo. More recently, two other Guard commanders and a former bodyguard to former Iranian President Mahmoud Ahmadinejad were killed, according to the semiofficial Fars news agency.

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A U.S. Defense Intelligence Agency official estimated that since 2013, Iran has lost eight senior Iranian commanders in Syria, including at least six generals.

"Replacing commanders is not easy," said a U.S. counterterrorism official. "Iran's deepening involvement in the conflict suggests Tehran will have to weigh the risks of propping up the Assad regime even more carefully."

Salami did not say how many Iranians are now in Syria. But a top American commander Tuesday gave senators a general estimate of Iran's military presence in Syria, as well as in Iraq.

"I think there's more than 1,000 Iranians that are on the ground in Iraq," said Gen. Joseph Dunford Jr., chairman of the Joint Chiefs of Staff.

"In Syria, we think the numbers are probably something less than 2,000."

Analyst: Much at stake in Syria for Iran

Matthew McInnis, a former senior analyst for U.S. Central Command, says he thinks Iranians are committed to the fight because they perceive vital interests at stake in the survival of al-Assad's regime. But he also said Iran could face a daunting death toll.

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"They are losing numbers of their best, to the point that they're going to have to rethink: Can we sustain this for years to come," said McInnis, who is now with the American Enterprise Institute.

"If this is just a four- to six-month campaign, maybe they can do it. But if this is going to be another two to three years of fighting, at this rate, I don't think they can sustain that."

But David Schenker, with the Washington Institute for Near East Policy, says he thinks Iran will not be deterred from supporting Syria's army.

"It's critical for them," he said. "They're going to see it through."

He said their commanders appear to be serving in forward-deployed positions, rather than advising from the rear.

"The Assad regime forces can't be relied on," he said. "They have to be controlled by the Iranians in a very hands-on way."

Indeed, McInnis said of the Iranians, "They're frankly guiding the entire Syrian front."