Advice needed: blocking up an old basement window

[Yan B.]

Looking for an advice on the most effective and efficient way to block up an old basement window.

Background: I was chasing leaks in the basement of my old house and found an old boarded-up window behind the drywall. I guess there used to be a window well on the outside that was filled with dirt. The window was originally cut in the foundation, there is metal frame still attached. No idea what's on the other side of that boarded up window, other than dirt and the A/C unit above it.

I consider myself fairly handy though I've never done anything with blocks/bricks/mortar. My goal is to block up this window without digging it out first. I think the first step should be removing metal window frame and any spikes/nails still in the wall. Not sure what to do after that. What type of block/mortar should be used? Or, is it better to leave this to a pro?

Thanks in advance for any suggestions.