Sir Keir faced criticism from both the Conservatives and Reform UK for not including a milestone on migration, with Tory leader Kemi Badenoch claiming the government had "no plan to control numbers".
In his speech, the PM said the government would reduce both legal and illegal, without giving numbers.
Taking questions from journalists after, the PM insisted he was not deprioritising the issue but said putting an "arbitrary" cap on migration did not work under the Conservatives.
He said the government had a "serious plan" to get the numbers down, including tackling the smuggling gangs behind small boat crossings.
Sir Keir added that border security was a a "foundational principle" that any government must deliver, while his missions were "on top of that".
He also denied watering down his party's previous commitment on green energy.
Labour's election manifesto promised "zero-carbon electricity by 2030", while his new milestone sets a target of "at least 95%".
"In terms of where we need to get to on clean energy by 2030, it's exactly the same as it always was," Sir Keir said.
"There's always going to be a mix but that is the pledge that we made two years ago. "
Badenoch described the speech as an "emergency reset", which "confirms that Labour had 14 years in opposition and still weren't ready for government".
Liberal Democrat leader Sir Ed Davey said "people want real change instead of a government simply moving the goalposts".
He said it was "worrying" there was no clear plan on how to ensure people could see a GP when they needed to, adding: "Pledging to bring down waiting lists while neglecting GP services is like robbing Peter to pay Paul."