Let's start by saying that all available evidence shows that SARS-CoV-2 (the virus causing COVID-19) is not made (and not even modified) in a lab. There's a very good Bio SE question on this, which I see no point in rehashing here.
So, that leaves us with a reduced theory that US has found a novel naturally occuring coronavirus and has weaponized it (to some extent) before using it in China and/or Iran. (There's a long history of [attempts to] weaponize anthrax for example, which is naturally occurring. But there's also a list of conspiracy theories that the US has developed new viral diseases like Ebola or HIV as weapons, none of which have had any real proof in support.)
But to stick with COVID-19, consider who else was seriously affected by it, namely Europe. Since Trump took power, the US-EU relationship has sank to a historical low. Going along with the conspiracy theory, you could even say that Trump planned to hit Europe too with COVID-19. (Although the sources you quote don't say that.) But then, why is nobody [serious] in Europe complaining about this alleged new US maleficence? It's not like they love Trump. Could it be that there's no evidence for Trump or the US being behind this pandemic? And that unlike China and/or Iran, spewing propaganda not backed by any facts is something that the EU and European national officials would rather not do? (Because compared to China and Iran, there's more democratic accountability in Europe, and press that's not just a government mouthpiece, so it could call into question unfounded claims.)
Another angle to consider is how sensible is it for a much more open country like the US to launch a biological attack that could backfire (via travellers) in its own territory. It is harder for more open countries to impose drastic, practically authoritarian measures to stem an infectious disease that causes relatively few casualties, but massive economic disruption. Insofar it looks like China has brought the epidemic under control better than some other countries have managed. So if the main target was China, that didn't work out so well.
But if you go even further in the conspiracy vein, Trump planned this in order to impose his own (even more) authoritarian/fascist rule in the US, while blaming China for the virus (the US administration basically did do the latter, at least it emphasized the foreign/Chinese origin of the virus). However, the "grand master plan" of infecting the US via China could have failed in quite a number of ways before having its effects in the US. Also consider that Trump was banking on a good US economy (which he often Tweeted about) to get him re-elected. And since insofar there's no real change in the power structure in the US compared to before the outbreak, Trump hasn't really gained anything obvious in terms of extra powers, but looks like he has lost his booming economy trump card... and he still needs to win an election this fall. (Another side effect of the COVID-19 crisis has been the plunging oil price, which hardly benefits US producers, with their higher production costs.) The health emergency doesn't seem to have caused any change in Trump's approval rating yet, either way. So you could say it was much ado for nothing if somehow "rallying the country behind the flag" was Trump's plan (to get re-elected). (Minor correction on that [March 28]: there's now a minor uptick in Trump's job approval rating. Still it's nothing like what Bush had on 9/11 etc.)
I'm sure one can come up with even more complicated scenarios, like a rogue operation I vaguely recall was included in an earlier iteration of the question, but ultimately one needs to invoke Occam's razor for these, as well as Hitchens's razor, i.e. ask where's the concrete evidence that the US is behind COVID-19 and what did the US (or Trump) gain from it in concrete terms?
And if you want an additional argument, China's most famous researcher in this SARS/coronaviruses domain,
"bat woman" Shi Zheng-Li (credited with identifying, after ~10 years or [re]search the natural reservoir of SARS' most probable relative in the bats of the Shitou Cave) is not accusing the US for the SARS-CoV-2 outbreak, as far as I could find out. Also of interest:
Near Shitou Cave, for example, many villages sprawl among the lush hillsides in a region known for its roses, oranges, walnuts and hawthorn berries. In October 2015 Shi’s team collected blood samples from more than 200 residents in four of those villages. It found that six people, or nearly 3 percent, carried antibodies against SARS-like coronaviruses from bats—even though none of them had handled wildlife or reported SARS-like or other pneumonia-like symptoms. Only one had travelled outside of Yunnan prior to sampling, and all said they had seen bats flying in their village.
In fact
By January 7 the Wuhan team determined that the new virus had indeed caused the disease those patients suffered—a conclusion based on results from polymerase chain reaction analysis, full genome sequencing, antibody tests of blood samples and the virus’s ability to infect human lung cells in a petri dish. The genomic sequence of the virus—now officially called SARS-CoV-2 because it is related to the SARS pathogen—was 96 percent identical to that of a coronavirus the researchers had identified in horseshoe bats in Yunnan, they reported in a paper published last month in Nature. “It’s crystal clear that bats, once again, are the natural reservoir,” says Daszak, who was not involved in the study.
Shi is in fact the corresponding author of that paper. So basically the top Chinese scientists don't really back up the claims of their foreign ministry.
Also worth reading about in this context, the 2016 SADS-CoV in which another bat coronavirus jumped to pigs in China, killing tens of thousands. (Shi's group was also involved in the research on identifying this virus.)
After more search, I see there is an ambigous/reserved statement on this matter made by another Chinese (high-level) scientist:
On February 27, renowned Chinese infectious disease expert Zhong Nanshan also questioned where the coronavirus had come from.
"The infection was first spotted in China but the virus may not have originated in China," Zhong said at a press conference.
There was no further elaboration in the snippet reproduced by China Global Television Network. But the Guardian wrote that:
Zhong later clarified his statement, saying that the first place where a disease is discovered does not “equate to it being the source”. He told reporters: “But neither can we conclude that the virus came from abroad. Only through investigation and tracing can we answer that question.”
It's worth pointing out here that Nanshan is mainly an epidemiologist and pulmonologist, while Shi is a virologist. Nanshan also held positions like the presidency of the Chinese Medical Association, which probably require more
political acumen/sensitivity.
