They don't call them the world power for nothing you know.
Without Nukes, the Iranians are holding up their defense pretty well and even fighting back, so imagine if they've got some of those in their Arsenal. The thing about these Nukes is that, you just might not know how much damage those things can inflict. And that's exactly what the US is trying to avoid, using Israel as a camouflage to do their thing and still appear to be appear to be ones proposing peace.
I see that the US isn't really interested in hurting nobody in Iran, thus their reason for proposing a peace talk, if Iran was so interested in putting an end to all the killings and attacks, then they'll just accept the treaty and then maybe come to reasonable terms with the US and all these would end in no time.
The aggressor here is Israel. There were supposed to be negotiations in a few days about Iran's nuclear programs, but Israel began to attack Iran, kicking some impotent personalities and destroying key infrastructure. If Israel stops attacking, I am sure Iran will also cease fire. But you don't expect Iran to go back to the negotiation when they are under fire.
I am surprised you think the US is not interested in hurting anybody when about 220 Iranians have been killed and more have been wounded. Israel was able to achieve it with the backing of the US.