If I want to become UK and US citizen then I have to give my Pk citizenship. But my question was if I obtain ILR for UK , my GC for US will be forfeit? And I will not be able to apply US Citizenship?
It's not a simple yes or no, as we don't know your exact circumstances, but obtaining ILR won't automatically forfeit your green card. You technically forfeit your green card if you settle outside the US, but as it has been mentioned before, you've been told by immigration officers that you're still eligible for citizenship in the US, which makes things confusing. So, it's not possible to say what the status of your green card is, although it SEEMS like it should no longer be valid. However, as I said before, my brother in law has also been told he's still eligible for citizenship despite residing outside the US and making regular trips back to the US. You'd need to consult an immigration lawyer, as you're getting mixed answers from immigration officers and people here who are familiar with green card rules.
IF you qualify for US citizenship and become naturalised, there's nothing stopping you from holding ILR if you reside in the UK. You can't retain ILR in the UK and reside permanently in the US, though. In my opinion, if you want to become a citizen of both the UK and US, you should find out your options, possibly from a lawyer, and if you qualify, become naturalised as a US citizen, reside in the UK and apply for ILR, then eventually get UK citizenship. There are strict residency rules for retaining ILR and becoming a citizen, so you wouldn't be able to apply for ILR in the UK, move to the US, retain ILR and later become a UK citizen. You would have to become a US citizen now (if you're eligible) and live in the UK with ILR until you can apply for citizenship. If you were to obtain ILR and move to the US, you'd lose ILR and would have to start the visa process over from the beginning if you wish to return to the UK, which is why it's advised to get citizenship before moving away from the UK.
It's confusing, I know.
You will be able to hold US citizenship and retain your Pakistani citizenship while living in the UK with ILR, as Pakistan does allow you to hold dual citizenship with the US, and you do not have to give up your Pakistani citizenship when you become a US citizen.
I'm unclear of your circumstances. It sounds like you're living permanently in the UK, but where you want to live in the future was unclear. Basically, as I said, find out your options for US citizenship and get it if you can. There's nothing stopping you from being a US citizen (if you qualify) and holding ILR in the UK.