Hezbollah has signalled that it may be ready to disarm if Israel withdraws from a disputed strip of land near the Lebanese-Israeli border.
Sheikh Qassim is the first to hint at compromise on Hezbollah's militia
Speaking to the Financial Times, a senior official of the Islamist group said it was ready to discuss disarming its militia with other Lebanese groups.
Deputy leader Sheikh Naim Qassim said the militia might become a "reservist army" within the Lebanese army.
Hezbollah, backed by Syria and Iran, is listed as a terrorist group by the US.
It appears to be positioning itself for the period after Syrian troops leave Lebanon, correspondents say.
UN Security Council resolution 1559, which demands Syria's withdrawal from Lebanon, also calls for Hezbollah to disarm.
In the Arab and Islamic world, it is seen as a resistance movement and credited with ending more than 20 years of Israeli occupation of southern Lebanon.
Hezbollah is the only Lebanese political party to maintain a militia since the end of the country's civil war in 1989. It still controls areas of the south of the country.
The disputed strip of land is the Shebaa Farms on the border with Israel.
The area has often seen clashes between Israeli forces and Hezbollah.
Lebanon and Hezbollah argue that the plot is Lebanese territory. This has allowed them to argue that the Israeli occupation continues and Hezbollah needs to remain armed.
Israel argues that the area is Syrian territory and says it will not withdraw until there is a Syrian-Israeli peace agreement.
The United Nations agrees with the Israeli view that Israel has withdrawn from Lebanon entirely.
Sheikh Qassim told the Financial Times that Hezbollah would discuss disarming "after Shebaa, but on condition that a credible alternative is found to protect Lebanon".
Hezbollah has organised a massive pro-Syrian demonstration in Beirut
He said a Hezbollah reservist army might be "a formula for co-ordination with the Lebanese army".
The deputy leader of Hezbollah insisted that the group was not militarily weakened by the withdrawal of Syrian forces from Lebanon, which is now in its final phase and due to be completed by the end of April.
"When Hezbollah needs military means, it knows how to get them... Israelis know our capabilities are much better than in 2000," he said.