In a rare meeting with a small number of journalists, Yahya al-Sinwar said Iran was "the biggest supporter" of Hamas's military wing, the Ezzedine al-Qassam Brigades.
Hamas, Sinwar said in response to an AFP question, was "developing our military strength in order to liberate Palestine", but he also stressed that the movement did not seek war.
Hamas has run Gaza since 2007 and received Iranian financial and military support for years before it was reduced following a falling out over the Syrian civil war.
"The Iranian military support to Hamas and al-Qassam is strategic," he added, saying the relationship had "become fantastic and returned to its former era".
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"Every day we build missiles and continue military training," he added, saying that thousands of people are working "day and night" to prepare for the next conflict.
However, he stressed that the movement "doesn't want a war and takes every effort to avoid a war".
"At the same time we are not afraid of a war and are ready for it."
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