'The evidence is accumulating that that's exactly what they are trying to do,' Mrs Clinton said in remarks in the Qatari capital, Doha.
She said Washington wants a peaceful solution to the nuclear dispute with Iran, but does not want to engage with Tehran 'while they are building their bomb'.
'I would like to figure out a way to handle it in as peaceful an approach possible, and I certainly welcome any meaningful engagement, but ... we don't want to be engaging while they are building their bomb,' she added.
Mrs Clinton is in on a three-day trip to Qatar and Saudi Arabia, aimed at enlisting broader regional support in a drive to stop Iran's sensitive nuclear work.
Mrs Clinton's aides neither confirmed nor denied suggestions that they would ask oil-rich Saudi Arabia to offer China supply guarantees in return for Beijing's support for new UN sanctions.
China currently imports much of its oil from Iran.
'Saudi Arabia has an important trading relationship with China already,' Jeffrey Feltman, Mrs Clinton's top assistant for the Middle East, told reporters en route to Doha, via Shannon.
Mr Feltman noted that there have been a number of recent visits between the Gulf and China.
'We would expect them (the Saudis) to use these visits, to use their relationship in ways that can help increase the pressure that Iran feels,' said Mr Feltman.
China appears to be the strongest holdout to sanctions among the five veto-wielding permanent members of the UN Security Council.
Russia has hardened its stance towards Iran lately, expressing support for warnings by Britain, France and the US that patience with the lack of progress in nuclear talks is running out.
In Doha, aides said, Hillary Clinton will discuss Iran and the Middle East peace process with Sheikh Hamad bin Khalifa Al-Thani, Qatar's emir, and Sheikh Hamad bin Jassim Al-Thani, who is both foreign minister and prime minister.
She will hold similar talks with other Muslim leaders attending the seventh US-Islamic World Forum, where she will deliver remarks.
Mrs Clinton will also hold talks on Iran with Turkish Prime Minister Recep Tayyip Erdogan, who is in the Qatari capital.
Turkish Foreign Minister Ahmet Davutoglu is due to visit Iran next week to push for a diplomatic solution to the stand-off over Iran's refusal to suspend uranium enrichment.
The sensitive process, which makes fuel for nuclear reactors but in highly extended form can also produce the fissile core of an atomic bomb, lies at the centre of Western fears that Iran is concealing a drive for a weapon, something Tehran strongly denies.
Iran insists its nuclear programme is for peaceful energy purposes.
Turkey, the only NATO member that neighbours Iran, insists the row should be resolved through dialogue, arguing that economic sanctions or military action against Iran would have a damaging impact on the whole region.
